If the game has functioning (or close enough so I can fix them) coin mechs and switches I will throw a few quarters in when I am out of credits. If the coin door is a horrid mess, that would be the last thing I would try to fix if it was rigged for free play (still would be on my to do list though).
Right now my go to game is 4 Square and I love having to add quarters because when you light the targets for special it feels even better to hear that knocker.