The golden rule is to avoid them, but sometimes you need to commit something that is not a real implementation, but only a clean-up, like old comments deletion, formatting rearrangement, and so on.

In these cases, it is better to isolate these kind of code changes in separated commits. By doing this you prevent another team member from running towards you with a knife in his hand, frothing at the mouth. Don't commit meaningless changes, mixing them up with real ones, otherwise other developers (and you, after a couple of weeks) will not understand them while diffing.