I've played tenor banjo - tuned CGDA like a viola ("standard" tuning) - and the key is irrelevant. (I don't now actually care for the tenor banjo - tenor guitar is much, much sweeter - but the same disregard for keys applies).

Sure, there are certain keys which are easier within that range - C, F, Bb, for example - but it's good for the fingers to be out of the comfort zone once in a while. I'm sure the skilled old-time style players can play endlessly in one key, but it's nice to get a lift up or down into other keys during an evening's music.

If I'm feeling particularly jaded during a long session, I'll start something off in F - that usually silences the massed melodeons. :-) Apart from that, I pay my taxes and don't drop litter.