I've stepped on them on occasion by accident. But for training crowding issues, a lot of quick left turns can help. If they don't "get" it, I'll kinda do a tap dance as if I'm aiming to step on their toes (never actually stepping on them of course) but just to get them to move their feet out of the way.
Also, if you're using food, make sure it's being given at your side and not in front of you. A common error is to reward the dog with the food a little ways in front of you and that teaches the dog to be there instead of at the side.