Overly Manly Man says:
it is a question of
a. Do you KNOW it was out. Did you clearly see it land OUT, no doubt in your mind?. IF so, call it out (except the 1st serve exception) No "hey let's play a let". No "didnt you think my ball was out?" Just say, "I saw my ball clearly out, your point" and walk to the next duece/ad court
b. If you are 99% sure it was out, but your opponent played it as in, then do not insult him and his judgement by second guesing his call. Let it go and let the game play on. Otherwise it turns into a "Chip and Dale" cartoon. "after you, No absolutely not, after YOU. I wouldn't think of it, after YOU, I absolutely insist, after YOU"