I think Nick's point is that you first need to define what a ground sheet is expected to do.
In some cases, you have rough ground, and you are trying to put a ground sheet in there firt to protect the bottom of the tent from puncture and wear. In some cases, the ground is damp, and you are trying to put a ground sheet down first to stop the moisture from moving up into the tent. In a few cases, you might be trying to use the ground sheet as extra padding for your sleeping, although that isn't efficient. It does tend to make air pockets in there, and that might help the temperature insulation situation.
If you let the ground sheet cover a light depression, and if the ground sheet extends an inch beyond the tent or shelter, then rain will roll off the tent, hit the ground sheet, and collect underneath in the center. This may or may not be desired, just depending on where you are and what your water situation is. If you don't want to collect water this way, then make sure that the ground sheet is completely underneath the tent and not visible.