Should I let the narrator (third person) describe that my characters are going to the bathroom (for a sense of realism and accuracy) or should I simply omit it as it as it adds no value to the storyline? I can imagine reading about characters going to the bathroom can get annoying for an audience, but at the same time it feels a little unnatural to write nothing about it.
Your readers only want to read a scene if it moves the plot forward, adds to a character's experience or inner life, or is just plain entertaining.
Realism doesn't mean a character gets up to use the bathroom just because nature came calling. It means that sometimes in the middle of a 3-hour meeting with no breaks, the anxiety that comes from needing to impress your boss is sometimes outweighed by the need to take a leak. And I imagine a long description of the relief felt at the moment might also be relevant, especially when it's interrupted by the sudden realization that he's not gonna get that promotion now.
Books aren't necessarily realistic or non-realistic, the moments they describe are. And I don't think an accumulation of details creates realism, I think the suspension of disbelief is what creates it. As soon as you've written something that absorbs your reader, you've created a reality for them. You've done what you set out to do.
Fun fact: James Joyce's Ulysses features a scene in which Leopold Bloom uses the bathroom and reads a newspaper. Very descriptive. The book purports to tell one day in the life but I can't for the life of me remember a second bathroom scene or a third or a fourth. Which means that in one of the most exhaustive and intimate novels of all time—where the 24-hour time frame was kinda the point (or gimmick)—we didn't get all the poops and pees.
Like anything else, if it's critical to the plot, or if it would be very weird to leave it out, then put it in. If it's unnecessary or there's enough passage of time offscreen to cover it, leave it out.
- If it's a hostage situation, everyone involved is going to be tense and focused for hours. The hostages may get desperate and sob that they have to pee, or the negotiator might be telling herself to ignore the urge because she has to stay on the line with the kidnappers.
- If the character has eaten something foul or been given a poison, then the effects on the digestive system are important. There was a scene in one of the VC Andrews books, the Heaven series IIRC, where the main character is given a severe laxative while at a fancy party but the bullies lock all the bathrooms, so figuring out what to do and how not to ruin her outfit is a plot point. (She uses a dry cleaning bag.)
- If you're describing the minutae of your character waking up and getting dressed, then using the bathroom is part of the morning ablutions. (as is washing hands, don't forget that part)
- On an episode of House, MD, "Top Secret," there was some weird side effect of his Vicodin use which prevented him from urinating, so he had to insert a catheter into himself. The viewer saw him on the loo, appropriately shot for broadcast TV, and then walking around with the catheter bag. A later nighttime accident (he's in bed and you see a dark splotch on the sheets) turns out to presage a typical House epiphany.
In any case, tailor the description to what's appropriate. "He went to the bathroom." "He used the toilet." An old-fashioned phrasing is "She performed her morning toilet," which covers all the washing and eliminating. But if she's been poisoned, "her innards rolled and cramped. Horrible noises rumbled from her guts. She felt like she'd been stabbed. When at last she was able to push out the noisome mess, it burned and splattered. She felt like she needed two showers to get everything off." The reader knows what is being described without actually, you know, getting the description.
Look at the books you've read - do they mention it?
If there's something unique about the situation - if your characters are extremely modest and are in a situation where they can't have privacy, or something - then it would add to characterization and could be included.
But if it's just a standard toileting situation, as suggested by your verdict that "it adds no value"? Leave it out.
This may be a case of taking the "show, don't tell" rule too literally. Writers should only "show" the important things. The rest can be "told" or ignored completely.
It would be interesting to write a story, just once, composed entirely of scenes taking place in a bathroom. People do a lot of interesting things while showering or taking a piss or a crap. They certainly think about interesting things while doing it. Make the reader try to deduce from their thoughts and actions what's going on in their lives outside the bathroom. Just a thought.
As others have said, if it doesn't advance the plot, leave it out.
A story doesn't have to mention every little thing the characters do. A story that did would likely be mind-numbingly boring. "Three ninja assassins prepared to burst into the room. The first assassin moved slightly to the left to give the second assassin room to open the door. Then the second assassin inhaled. The third assassin scratched his ear. The first assassin moved his left foot slightly to the left. The second assassin grasped the doorknob and turned it. The first assassin's heart beat four times. The third assassin had a slight itch on his left knee." Etc. Okay, there could be times when such details would be good and appropriate, like if you're trying to emphasize that someone is bored while waiting, or to emphasize the human frailties of a character while he is performing some dramatic action. But barring that, it would just be boring, tedious detail that would bog down the plot to a ridiculous extent.
Add to that, going into detail about someone using a bathroom would just be unpleasant. Yes, I'm aware that people have to urinate. But I don't want to read detailed descriptions of the process. Again, it could be appropriate if the point is to emphasize the mundane tasks of human life. But usually the readers reaction would just be "yuck, can we get back to the clues to the murder".
Again, it is widely believed that you need to describe everything. But when you feel like it is necessary to add to your plot, go ahead, write about that poop! If not, you're not going to want to add it. When you add unnecessary details about everyday things, people might not actually want to read that scene. As an author, I'd prefer if you feel you need to add that for suspense, go ahead. But don't use the bathroom scenes if it's just going to pull your story down.