Bluegrass musicians play "Old Joe Clark" in the key of A and use a G chord in the B part. Fiddle tunes are played differently in different styles of music, and this tune is no exception with many variations itself. Old-time musicians will likely stick with the E chord in the B part instead of the G.
Whatever the fiddler does, that's what you play along with. Charts and tabs are nice, but bluegrass music really came about in the aural tradition, so as soon as you can play without charts, you'll start FEELING where to change and where to put in a short or longer bass run.