I use both forms: I didn't use to (or never used to for emphasis) and I used to not, but for me there's a difference between them.

For me, "I didn't use to" just mentions a past situation, but "I used to not" implies a conscious intent.

Thus, "I didn't use to watch soap operas but now I find them addictive." That's just the way it was, I didn't watch them.
But, "I used to not watch soap operas" would imply that I'd made a conscious decision not to watch them for some reason.

Thus I would always say "I never used to like fish" rather than "I used to not like fish", since liking is not a matter of conscious decision.

Bingley

Bingley