To the contrary, being a fitness trainer, I am constantly telling my clients to look at themselves in the mirror to insure proper form!  When they are setting up to perform an exercise, 9 times out of 10, their posture needs correction!  If one isn't aware of how they have been patterned to stand over the years, they will most commonly hunch their upper back, with their chin protruding, shoulders rounded!  How can you possibly know this if you don't see yourself doing this?  I have seen people look down, look up, or tilt their head when performing an exercise! Performing any exercise in this poor position just sets you up for injury down the road!  MIrrors in workout rooms are most definitely necessary and do aid in improving performance, when used for this reason!