A skidding wheel has less traction than a rolling wheel, so I would say that it does not help the braking. However, if you are upright and going straight, it won't hurt all that much. It is very easy to lock up the rear wheel under maximal braking, since so much of the traction is transfered to the front wheel. I have done it just about every time I have had a true panic stop situation. I can avoid locking the rear wheel just about every time I PRACTICE maximal braking, but that is sooo different than when you are actually going to collide with something.