Always be on top. It’s one of the tenets of combat. Gravity is on your side if you’re on top. You have the weight of the world, plus your own weight. Even if I’m in guard, I prefer to sweep into a top position, instead of submitting from the guard. If I pass in a guard, I’m also on top. There are much more submission options from the top position. Side control, mount, knee on belly, kesa-gatame, all have submissions. There are very few submissions from bottom. If my opponent is under me, he must fight my pressure and my submission attacks.