Just using manners in general is always a plus in my book. However, you would be amazed at how much saying “thank you” can improve anyone’s day. For whatever reason when I use manners with people I don’t interact with on a daily basis it catches them off guard a little, but generally improves the interaction. This happens often enough that it saddens me a little bit when I realize the lack of manners used in general.
My ultimate point though is that manners help greatly in high pressure situations. When working in the pharmacy and every one is running at full tilt and the wheels of the wagon are about to spin off I say “Thank You”, as often as I can. I find that it kind of calms everyone down a bit because I am acknowledging even that smallest contribution everyone is making in what is probably a shitty situation. That way when sanity finally returns we need less time to decompress a bit and get back to kicking ass and taking names.