This has just happened to me. A player quit for no apparent reason, (did not explain why) and so I naturally red thumbed him (although he will get a penalty to his reputation anyway). I am not inclined to red thumb people as far as I can recall. However the red thumb to green thumb ratio was invoked, and it was suggested that I personally red thumb him. But this was not my true opinion. I don't think people in general should play with someone who quits without an explanation. Its possible of course it was an emergency, but I can't assume that without evidence.
In terms of giving green thumbs out, I sometimes limit, because if I give everyone who isn't bad a green thumb, I become confused between whether I enjoyed playing with this player or they just were neutral. But often I will give a green thumb if player was polite, and said good game or communicated in some friendly manner (or even communicated at all!) Amazingly a lot of people don't even bother with this (sometimes in rapid fire games its understandable I suppose). But I'm not giving anyone a green thumb unless I had some response from them. And if player was of some real distinction, I will make a note about them (good or bad).
However I think maybe the ratio of 5 greens to 1 red is too high. Maybe lower it to 3 to 1?