The right key for a bluegrass instrumental will be determined by the lead instrumentalist. Some common instrumentals have universal keys, such as "Old Joe Clark." It's generally played in A, though you may find a few folks who play it in G. The right key for singing is determined by the lead singer.
I frequently use a capo to play G chord positions in A (second fret), Bb (third fret), and B. I usually personally prefer that sound to the open A chords. But in "In the Pines," the open A really speaks to me. Experiment and see what you enjoy for your voice and your hands.