but I wish that B, C, D, E and G didn't sound the same when called out by a fiddler or accordianist when they're about to change key! I once tried to instigate a system whereby the lead player called out something like 'Cat' for C, and 'Dog' for D. It didn't work.
When leading, one of our accordion players just tends to let out a sort of 'whoop!' when it's time to change tune, so if you don't already know what key the next tune's in, you're b*gg*red - that's why I like to know the keys in advance :-)
Another reason I like to know all the keys in advance is that I sometimes use a capo for certain tunes, as I can do different things to make it more interesting. But of course I don't do this for sets of more than one tune, if it would mean having to remove the capo in the middle of the set. We do play some pairs of tunes where I prefer to use the capo, as I find I can do more interesting things, e.g. I sometimes capo on 2nd fret and play G and D instead of A and E. Sometimes I might play the accompaniment with the capo, sometimes without, to make things more interesting for me, if no-one else.