I'm failing to see how filing with your left hand is different to filing with your right hand, other than the fact the file is in the other hand. When the client gives you the other hand it's the same as the mirror thing you're talking about. But even then both hands are filed in the same way. I'm right handed, you probably guessed. Sometimes i'll use my left hand depending on the position of the clients hand. Is it not the same as filing your own nails then having to do the other hand? It's just practice. In fact it's not like doing your own, that's harder because you're having to use the hand you wouldn't normally.
