ORIGINAL: TFOX
You will not see enough of a difference in most hunting situations on flat ground to ever worry about the slight difference.
If you have stands setup on steep hills where you might be shooting downhill,then learning how to get the yardage will be a huge help.kwilson gave you a good pointer,range the trees straight out and that will get youreal close.
+1 to
this. A slightly higher POI isn't such a bad thing really when you are considering that the angle of entry may require your POI to be higher for a "good" hit on the vitals. And if you're really concerned, the ranging trees at eye level and then considering what the distance is to help you shoot is helpful.
Practicing from a treestand IS important if you've never shot from one, or don't really feel comfortable shooting from one, but the yardages aren't really going to come into play in most situations.