If I'm playing against someone that is generous with the line calls, I'll return the generosity. By that I mean I'll hit a generous amount of serves/ground strokes/volleys as close to the lines as possible because hey, my opponent has widened the margins for me!
But seriously, if someone is letting a few go in my favor that were most likely out, I'll do the same with them.
If someone is calling my balls out that are in, I'm not so quick to think they are cheating me. I've played against more than one player that is somewhat aloof when it comes to watching lines or just doesn't pay attention well.
If they are cheating though, I remember Jim Courier had a funny story about that. He told a story about some little brat he was playing when he was a junior that was calling all sorts of good balls out...really trying to cheat a win out. Well, the next time the kid served into the middle of the box, Courier called it out. When the kid threw a fit, Jim said, "We can do this all day." I guess the kid straightened up!