Vic Braden stressed net clearance to reduce errors. I think he used 3' and 4'-6' as reference points.
Many other coaches use this theory too.
Robert Lansdorp talks about hitting very hard drives 1.5' to 2' feet over the net. He puts a court roller on the net with a tennis can stuck on top as a target and has pupils hit hard drives at the target. The target is about the right height 1.5-2'.
Occasionally, televised tennis will show average net clearance for a pro match. I have seen Nadal's and Djoko's average be around the 3' marker.
I have also seen Oscar Wegner and other coaches advocate a 3' line above the net as a target as a good baseline rally target.
Clearance is not as important in doubles as you are striving to get the ball low much more frequently in doubles in my view.
I think aiming aggressive hard shots 1.5' - 2' over the net is fine and aiming slower rally balls and defensive shots 3'-5' over the net in singles are reasonable guidelines. You can also think in terms of court postioning. If you are 6' or more inside the baseline, drive it in the 1.5' - 2' window, if you are near baseline or behind baseline, increase the margin.