You need to learn to feel when you are on the wrong lead. Picking up the wrong lead and having to correct it will cost you in dressage.
In a movement requesting a canter and the wrong lead is picked up (assuming all esle is fine--body position etc) and NOT corrected before the next movement is called will not get higher than a "3" from me. Correcting the lead within the movement space will get you a "4" from me.
I would not give a "5" or better as a "5" means sufficient and clearly an incorrect lead is not sufficient.