First of all, you can give opponent(s) the benefit of the doubt and play balls slightly out - happens all the time. If a ball is way out, more than an inch or two, it really should be called "out". With the exception of the first serve most balls that opponent(s) play that are "out" only hurt them - they don't gain any advantage.
In the instance where opponent(s) play everything, just forget about it and move on. Don't get in the habit of over ruling/making calls other than your own. As you move up, this won't be a problem so relax and just play.