here's the deal, different people have different thickness chests and different lenth arms. A skinny guy touching his chest is different from a fat ass touching his chest.
How far you lower the bar depends on your body type. For skinny people (I'm one of them) lowering hte bar all the way to your chest loads the anterior deltoids too much. THis is dangerous because the anterior deltoids were not designed to support this much weight at that angle. You are also not benefiting your chest by lowering it that far. I agree with lowering the bar slowly and in a controlled manner with a very slight pause at the bottom but the distance needs to be judged by the lifter. You need to be intune with your body and feel at what point the load is being taken off you're pecs and being placed on your delts, at that point, stop and begin the concentric portion of the rep.
All you guys that feel like you've ripped your armpits the day after you bench are going to far down and hurting your delts. The only reason for not going that low is there is no additional benefit to it and it MAY cause injury.
Powerlifting contests require touching of the chest because it's a standart by which all contestants can be measured equally, or so they think. A thick chested guy with short arms has a hell of a lot easier time touching his chest than a skinny guy with long arms. Doesn't seem like a fair method to me.