In the jazz world there are certain standards that every jazz musician must know. All Of Me, Autumn Leaves, Girl From Impanema-if you are going to a jam session you better know these and many more. As far as keys go with vocalists, there are certain keys that you don't ask a musician to play in, or the vocalist loses respect for the singer. Key of B for example.
I can see where there are hymns that everybody in a gospel church has been singing forever that are kind of like these songs. Some of them I know, some I don't.
With the hymns I grew up with, I can play those pretty much in any key.
The way our church starts, the deacons always start singing some songs. Some of the little what I call devotional songs have a different melody than what I grew up with but the same words. Plus, the deacons don't give us the time to do the standard turnaround at the end of the song, so we have to adapt to them. They are wrong for that, but they are the deacons and you don't want to make them look bad.
My philosophy as an accompanist to a singer is if you mess up, it’s my fault. If I mess up it’s my fault. It is my job to make the vocalist look good no matter what. It’s served me well, and they always call me back.
There are a some headstrong leaders out there that are going to sing the song in whatever key they start it off on, and they don’t necessarily have perfect pitch. It is a touch of professionalism to get the key ahead of time from the vocalist and the pianist could give a intro to establish it, but doesn’t always work that way.