Generally, I do 2 buds on a bench graft. Exactly like people said above, the second one is just for insurance. In winter, the shorter of the two gets clipped off.
When field grafting, I am starting to use larger and larger scions. I'm using a technique I can't remember the name of, but it's basically grafting low on the tree instead of above the nurse branch. Then you cut off everything above the graft. This gives the benefit of having far more top growth while reducing the problem of the rootstock throwing new growth.