I agree with the cover aspect. If there is split in the tree with good cover and shootabilty at 15'. Why go to 25 in the open with less shot. Look from the ground and see what's there for a back drop. There is no set rule. As a hunter it's all about adaptability to your surroundings. If the deer aren't moving in the tall tress. You can't not try to get in there because you can't be 20+ feet off the ground.