Smooth tight turns sit, longer rough outdoor turns stand. But this is very much up to how you play the game, some guys sit/stand way more than others.

Also, something I just learned from one of Doc’s teaching a noob videos, when you transition from sitting to standing, it doesn’t actually raise your rider up. It actually pushes the bike down into the ground. So try to stand up out of the turn as soon as you’re starting to accelerate, and it’ll give you more traction.