kneeling posture in Bjj
 

When you are in somebodys guard or not, or whenever you have a knee down; should the top of your foot be flat on the mat, or should your toes be in a ready to spring position? i have just always wondered which is best, since I came from a wrestling background and I've always been told by my coach to keep them flat; the reason being is that having your foot raised up like that gives the opponent a potential place to gain levrege by grabbing it, and you have more balance with it flat. What do you more experienced guys think?