Certain breeds are supposed to carry tails high and that allows you to hold the tail up when you are stacking them. When you stack up a dog they usually relax there body and the tail is relaxed and lays down. However you have to hold the tail up so the judges can see the tail set. You can't tell the set of the tail when its down....thats why its held up.
Our Amstaffs we had are supposed to carry there tails like a pump handle. On that breed you never touch the tail because it's supposed to relaxed down and never carried over the back. Certain breeds are different.