I will call my own shots out if I'm sure. The most common scenario is attempted winner / passing shot down the line, where my opponent is looking across the ball while I have a good view of it. Most opponents will give me benefit of the doubt on a ball that is slightly out, but if I see it out, I will call it on myself.
Having said that I would not be as quick to call a 2nd serve out, because typically I am still recovering from my service motion when the ball lands. I'm not tracking it through the air, and I don't get a very good look at it. So unless it's obvious to me, I'm going to rely on the opponent(s) to call it.