"truth can only be handed out sincerely" - I sincerely can't see why that is true. It's perfectly possible to hand out truth malicuiously or hypocritically.
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Most people in my experience seem to say "thank you" to bus drivers, or indicate it by some gesture. That may partly be to do with the fact that most people on buses seem to be older, using bus passes, and were brought up that way. (Though of course we used to have bus conductors, and the drivers were locked away, so the precise etiquette was a bit different.)