My labels are put on as the last step in the process. Sometimes I don't know what I want to say on the label until the quilt is ready to give away. Mine are sewn on by hand, but I have heard some people say that they let the machine quilting go right through the label, having the stitching land where it may -- the point is that it makes the label more difficult to remove without leaving a noticeable absence in case the quilt gets into the wrong hands.
I think the standard place to put a label is on the lower right hand corner back side. That is, when the quilt is facing you as it would be on a bed or wall, then when you lift up the lower right corner, you will find the label.
Having said that, I place them wherever I need to near the bottom. Sometimes my positioning is for design purposes, and other times it is to cover up a blemish of some kind. Ha!