Is it better for first person POV to be treated in the same way as third person POV meaning that the POV person withholds opinions? (So the reader can form them?) Or is it okay to establish conflict via first person POV opinions? Is that a sloppy method?
You can expose opinions in third person POV. In "close" third person you're extremely likely to expose the third person POV character's opinions. And even in third person omniscient, you could have an omniscient voice with an opinion. The Chronicles of Narnia, for example, is full of narrator opinion.
In first person, it would be unusual--not impossible, but quite unusual--to avoid having the first person's narrator's opinions.
As others have explained, if a story is written in first person, the readers expect to be privy to the POV character's opinions, thoughts and feelings. This is true of third person limited too, and even an omniscient narrator would get into characters' heads.
However, this does not preclude the readers from forming their own opinions. They might, after all, disagree with the POV character's evaluation of a situation, or even their approach to the main conflict. The POV character might well be mistaken about something, and perhaps be made to pay for their mistake.
There is nothing underhanded about using the POV character's inner thoughts to fuel conflict. It is, in fact, quite commonly done. Consider, for example, Jane Austen's Pride and Prejudice: the source of the main conflict is the main characters' mistaken opinions of each other.
Preferably, in first person POV, you're completely transparent with precisely one person's opinions and thoughts; the POV character's. Everyone else's, though? They're as opaque as any other human is to us.
As such, you get the experience of 'living through [POV Character]'s eyes' and while you will know everything about what they think, you're left to form your own opinions about what others think of the POV character, schemes that may be going on without their knowledge, et cetera, et cetera.
The Animorphs series of books provides us with another excellent example of the need to be privy to the main character's thoughts. The series has a rotational cast of First Person narrators who frequently have to describe themselves experiencing the emotions of an animal (all the characters have the power to turn into any animal they have previously touched. One of the rules of the process is that the first time morphing into a new animal can be overwelming and the narrator frequently finds the animal's instincts overriding their still human personality) as well as the sensual changes. There's a very good bit in one of the books where an alien experiences a human mind for the first time and describes the sensation of self-doubt that was never something her race had a problem with.