The 3/8 or 1/2" gap at the floor, with the ceiling installed first, gets covered with the base molding. I can't see trying to cut or snap 3/8" stips of drywall.
Having installed & finished drywall for a while many years ago, I never understood installing it horizontally, even though most reisdential work is done that way. I was doing commercial work at the time. One vertical seam to finish every 4' as opposed to a continuous horizontal seam at 4' and verticals every 8'.