open G tuning has 5th tuned to G and played open (what we usually hear).
5th at A will match when playing out of key of D or A.
The 5th string is normally a drone note. The best drones are usually the I or V notes in Western music. Using a II note as a drone makes for some "different" sounds as you progress through the chords of the key you are in, just to try and keep it easy theory-wise.