That'd sure come in handy for me.
This is a question of "voice." In general, characters are expected to speak largely as real people speak, and real people often speak with non-standard grammar. However, if it becomes too hard to read, modern readers will find this annoying. At one time it was popular in American literature to write entire books in "dialect," but a modern author would be expected to just sprinkle enough dialect into the dialogue to give some flavor.
As far as the narrator, if the narrator is a character (first person), the same rules apply. If the narrator is not a character (third person) then you would generally use standard grammar. An exception to this is if you want to give a particularly strong sense of your own authorial voice. You'll want to be cautious about this, however, since readers will tend to get tired of this even more quickly (unless they happen to share your own dialect).
Honestly, unless you have purpose behind your misspellings/strange grammar, stick to rules where you can. I myself have a few stylistic exceptions to grammar, largely in dialogue (if someone has a tendency to shorten words/has a particularly strong accent, I may make the dialogue more phonetically spelled). Even in my case I generally play on the side of cautious.
Sometimes, ignoring grammar and spelling can be useful for a style. Most of the time, though? It's a mess. The rules exist for a reason, after all.
Here’s the truth, it matters, but it isn’t Gospel. All communication, including your grammar and spelling in your writing conveys the level of effort you put in, and your desire to communicate to your audience. That said, when you can still convey your effort and ability to communicate through other means, you’ll quickly find there are many rules you can break. Language is always adapting, but it isn’t changing from those who are lazy with language, but those who creatively take risks, and adapt to communicate more interesting and compelling content. Good luck!