From all this discussion it appears I'm probably in the minority of players who have never called a hindrance in league play. I've been playing USTA and our local league regularly since 2005 and have yet to call a hindrance on an opponent in either doubles or singles. In fact I don't think I've even warned an opponent who I thought was exhibiting questionable behavior that what they were doing (e.g. yelling while the ball was headed towards us) might be construed as a hindrance. I have however discussed it with a doubles partner when I thought their actions might be considered a hindrance by the other team. The flip side of that coin is that I've also never had an opponent call a hindrance on me or my partner either.
A Head Microgel Radical, a Neos 1k, and a Silent Partner Star. Life is good.