I always try and give the benefit of the doubt to my opponent and how the ball lands. If I honestly can't tell, I don't call it. It's gotten me into trouble a few times. I'm really, really bad at being able to tell if a ball is out on a sideline; especially if I'm on the other side of the court.
Last year I was playing a match at Sectionals in St. Louis and my opponent served to my partner out wide. I was looking into the sun and couldn't tell at all. My partner played it, hit a winner, and the guy serving freaked out on us for playing an out ball.
I tend to think bad calls/no-calls even out over the course of a match. Everyone makes them, if if it's not intentional.
If I feel like someone is honestly hooking me, I'll say something. If it continues, I'll call the next serve the guys hits out, no matter where it lands. I've only had to do that once, back in college.
"It is not enough that I succeed...everyone else must fail."