I think ANY dog's tail can bruise some people. Why do you assume that because you don't get any bruises from your dog's tail that no one ever would get a bruise from a dog's tail? I've gotten bruises from tails before, not from rotties, because most the rotties I know don't have tails(stubby things, not tails), but other similar dogs, including mixes, or labs, or other larger sized breeds have indeed bruised my legs, and arms in some cases.
I'm not saying to dock a dog's tail to prevent bruising your legs, but still, it can happen, and tails do indeed swipe stuff off tables in some cases too.
Again, if it's someone choice to dock a pup's tail, and it's not illegil, then they should go for it. If you don't want to dock your dog's tail, then leave it.