Guitarrist here, played hundreds or thousands of standards gigs. It's true that you can assume that good musicians will play a tune in any given key. However,

Guitarrist here, played hundreds or thousands of standards gigs. It's true that you can assume that good musicians will play a tune in any given key. However, I think in the real world nobody plays equally well in all keys. Also, even for instrumentalists, things don't lay the same way in different keys. Sometimes people intentionally call tunes in uncommon keys to force them off the beaten path, which is telling. I think singers should be able to sing in all keys, just like everybody else. That said, in a jam session, student practice or other informal setting it's recommended that the singer brings sheet music for everyone for other keys than the "usual one". Also nowadays people will reach for their cell phone and "real book" app. On the other hand, I won't be caught reading standards off the paper in public, so there's that too.