This question is from Minecraft 1.1 to 1.6, when getting wool from sheep required hitting them, rather than killing or shearing them.
I haven't read anything that confirms my suspicions, but I can almost swear that after months of playing Minecraft and harvesting a lot of wool for creations (and trying to make the most out of lapis back when you only got one dye per ore) that hitting sheep in different areas are more likely to yield certain quantities of wool.
Maybe it's confirmation bias, but from all my shearing, I feel that hitting them in the head is more likely to yield 3 wool, in the side is more likely to yield 2, and hitting them from behind is more likely to yield 1. Has anyone else observed this behaviour? Is there any confirmation from Notch or another developer that no matter where you hit a sheep, there's an equal chance of getting 1, 2, or 3 wool?