I believe it's whichever you feel more comfortable with, rather than whichever you write with. I think part of my problems in the past have been that I change hands - I learned fingerspelling right-handed as a kid, but my BSL instructor was left-handed so I got myself in a muddle copying him (You'd think mirror image, right? No). I just used whichever hand felt correct at the time. I got my left hand smacked more times than I could count.
I'm now trying hard to stick with left-handed. Right clearly didn't work for me. But now I'm having to undo three decades of right-handed fingerspelling.
Since your post was four months ago I assume you figured it out for yourself, but I thought I'd reply in case anyone else was wondering.