I concur with saying something at the time.

On occasion, the problem can be fixed by the FA. Then all the reasons previously stated, along with a restatement that this saves a fellow traveller next to occupy the seat the problem AND you demonstate that you cared at the time, not later as if someone had told you to call and ask for stuff. Finally, consider how often you call with complaints and insure those times are worthwhile enough to do so.