Well, it depends on the situation. If you're in a hairy situation like a steep greenside bunker, you won't want to adjust your impact spot, because you'll have a tendency to hit it fat. The benefit of the flop shot is that you can produce a high, soft landing shot in situations where you can't produce a great deal of spin. However, if you've got a situation where you're going to hit a flop shot from a clean like, like fairway or fringe, you could certainly produce a little more check by hitting down on it.
Hope that helped
-Dylan