Opinions are def mixed on that question. I'd like to wait, because if it's so unstable that it won't stay variegated on its own roots for a couple of years, you're probably wasting time grafting anyway. Also, at this size, there can't be a lot of root, so I would want to get that established before cutting. Best understock is davidii, eventually (as I expect you know). Some people graft as early as poss, to get on stronger roots, like for a young dissected JM, since they're notoriously weak. But I'd guess the risks outweigh the rewards on this one.
