I usually always try to get to at least 20 ft, but will hunt as high as 30 ft. The canopy does affect how high you can hunt. I feel that if I hunt any lower than 20 ft, then you are more apt to have any movement spotted, and you are more likely to get winded. Obviously the higher you hunt, the more your angle changes for bowhunting.
I have a buddy that is scared of heights, and he only hunts about 12-15 ft off the ground. He sticks out like a sore thumb and is always getting busted.