Back when I was still using the men's bathroom, I didn't really care. I preferred to sit down because it's cleaner (you actually get to wipe that last drop off instead of the incomplete "shake" method, and there's none of that gross splatter,) but I did do it on occasion if I was in a hurry or if the stalls were all pee-splattered. (Which happened a lot... guys can be real slobs in the bathroom.)
I would NEVER stand up in a women's room, though, and I hope that's not where you're doing it. Being allowed into that bathroom at all if you're pre-op and don't have a doctor's note or anything is a privilege. So if you're going to use it, you should probably play by the rules, otherwise you're just asking for problems.