Fed might not have the same grip for his volleys and his slice backhand groundie.
If you have time to take a swing at the ball, why use a sliced backhand? Why not hit a topspin backhand, with the grip change and shortened backswing?
Any player who faces a real first serve uses the conti grip for all his returns until he get's used to the pace. Once he's dialed, he can use whatever.
If you can use forehand and backhand grips for return of serve every single time, you are playing low 4.0 tennis against weak servers.