I suppose you could sit down and crunch some numbers to figure out less throttle in lower gear vs. more throttle in higher gear but it would be theory anyway. The real gas saver is the right foot and its associated pedal.

If you're really worried about it, drop some speed going up the hill, and switch the AC off. Then on the back side of the hill, recoup your speed and switch the AC back on. Down shifting is a judgement call, use whichever gear lets you press lighter on the pedal.