I cant really give you an answer that I can back with 100% integrity. I know for shooting with a gun that may be a farther shot, say 100 yards, and if you are shooting uphill or downhill, you are supposed to hold just a tad lower than you would if you were shooting level ground at 100 yards. I'm sure the angle does effect how much to comensate. You say you hold slightly high on a mild uphill shot, if it works dont change. But going on physics alone, whether its uphill or downhill you should compensate by holding a hair lower. In a bow situation you're looking at 30 yards and if its just mildly uphill I'd hold dead on. This "holding low" application is more geared towards an extreme angle uphill or downhill.
Since you've had experience shooting as you have and never had a problem apparently you did something right, so I wouldnt change a thing.