I generally agree with Jeff on this question, at least as far as using trees on the tee shot. If we built a hole with a tree blocking the left side and more trees down the right, so that everyone had to hit a draw off the tee or go in the trees right, that would be architectural brutality. (However, it might be okay if you could lay up even with the tree and still have a reasonable shot home.)
On approach shots, though, I feel it is fine to "shot test" the golfer and say that if you can't hit a draw or a fade to counteract a slope, you may not be able to get at a certain hole location and leave yourself a shot for birdie. However, I generally wouldn't use trees to accomplish this, UNLESS I had given the golfer plenty of room to place his tee shot to one side of the fairway, so he didn't have to play the fade or draw to get around the tree on the approach.