Clarence White preferred mahogany dreadnought for leads and a rosewood dreadnought for backup. He used an old D-18 to play the leads on Appalachian Swing, but the D-28 now owned by Tony Rice for backup. Doc Watson (although he was not strictly a bluegrass player, he did dabble in that genre) also preferred a mahogany dreadnought.
Bluegrass is one of the few genres where "plugging in" is still frowned upon and although many Newgrass players have started using pickups, the traditional players still prefer to use a mic.
I recall a Tony Rice interview where he said, "Any interviewer who asks me what kind of pickup I use has obviously not listened to my music."