From my experience at embarrasingly pathetic recreative leve the most important factors are disguise and tactical decision on the placement. Also, not thinking that the point is won and getting ready for the DS return is an important factor.
Also, disguise is an element that comes along with having a wide range of shots. For example, my opponents can't tell if I'm attempting a backhand and even forehand slice to the base line or a dropshot, so sometimes they just stay back when it's a DS or come to the net when it's a long slice. My disguise from TS forehand or BH is BAD, but I use the slice forehand quite often (wicked sidespin).
Now, answering, I believe that anywhere above the net height is great if you can give some underspin with minimum loopy trajectory.