Tipping for the Holidays

The mailman is easy - our guy Aris is very conscientious and never seems to forget that we'll be away on vacation and carefully keeps our mail for us. We normally give him a $10 or $20 tip when we return and at Xmas we give him a card with $25. He always seems appreciative and gives us a thank you card.
The paper person who we never see, leaves a bit to be desired in her service to us and so she gets $10. This year she gave us a note saying that she is moving on and won't be delivering beginning in January. Hooray!
Our dilemma is the trash guys. Sometimes 2 other times 3 of them. Usually there's one guy who remains the same and the driver we can't see so we're not sure but I'm under the impression that the one guy who seems to be on our route every time is in charge. I will often give them bottles of water, particularly in the summer when it's hot. Once when I was fooled by there being 3 and I thought 2 I told the guy that I needed to grab another bottle and he said not to worry, he's fine. so does that make him in charge? Who knows? Maybe the 15 - 20 times a year I give them water is enough for the year? I'm not sure what to do about them. I hate to ignore them.