I treat trees as if they were people. If an old, fat tree can stand its own in the middle of an empty field, then no need to add any more. If a skinny sapling is better off in the company of friends, then let it be so. However, in the end if a tree or group of trees fail to add anything to the composition or leave your shot a mess of confusing twigs, then I'll either move to a better spot or forget it altogether.