it is always a good idea to have three songs ready to hand out to the players as soon as you walk into a jam session. it can be very disconcerting to be called to the bandstand before you have had the chance to sit down and organize yourself. believe me it happens a lot and it is unnerving. consider this: place the three songs you intend to sing (a swing, ballad and latin) at the very front of your folder -- this means you should have 9 charts to hand out. then if the band was playing a ballad or a slower latin, you are prepared to sing a swing, etc.
you can always change your mind about the songs you've chosen once you get to the session if you get the chance to sit down and order your beverage before you get called on to sing.
-armonica