I have only seen 5 string fiddles used for Indian classical music - there is a famous Indian violinist called simply Shankar. My friend Martin has a six-string electric fiddle which he had made recently - I will ask him about it. Treating the low fifth string as analogous to the intervals on other instruments, it would be sensible to use a tuning of B F# D A E, but I suspect that CGDAE would feel more familiar. I used to play five-string guitar for tunes, and I tuned that D G C G D, but that was so that I could read off fiddle music fairly easily and play sympathetic bass as well.