And if, like me, you mostly play for yourself and don't have to be in tune with other instruments, the rule is always "Find a pleasant tone," i.e., one you can sing with.As Joe Clark said, "There ain't no notes on a dulcimore, you just plays it."
Bless all the old mountain music-makers for providing an instrument that even a musically challenged person like myself can enjoy! My advice would be don't get too hung up on the various "modal tunings"--and remember to just have fun with it!
Lin