Shelf, elevated rest, your hand- there seems to be some misunderstanding. When shooting off your hand, the only incosistencies would be in the bow's action, not your shot. Slefbows aren't perfectly consistent, so it's not as big a deal as with, say, a modern recurve.
Likewise, an elevated rest means your hand will just be slightly lower than otherwise, the aim being the same. However, with an elevated rest variations in cant will throw off your form more dramatically than shooting with the arrow closer to your hand. Likewise, some moving targets only give you time to "point", and here your brain my get in the way of your practice. Other than that, everything about the shot would be the same.