PHJim, G-D-DD is not a tuning I've heard before. To get to G, I usually tune D-G-D (as I only have three strings). I'll have to see if I can coax that bass string down to G and the middle up to D to see what that sounds like!
As far as strumming only in one direction, that may work for someone who's never played the instrument before to get a good tone and to play a simple tune. But in order to play fiddle tunes at speed with other instruments, one pretty much has to strum both back and forth (or forth and back according to your preference). I angle my strum slightly up and outward to emphasize the melody string, but to also make sure I keep the rhythmic drone going on the middle and bass strings.