[as_bntd]

There's a difference when use is preceded by did:

> The problem becomes a little trickier in constructions with did. The form considered correct following did, at least in American English, is use to. Just as we say "Did he want to?" instead of "Did he wanted to?," so we say "Did he use to?" instead of "Did he used to?" Here again, only in writing does the difference become an issue.

> While in American English "did used to" is considered an error...

Personally, didn't used to looks jarring, but it made me open the article so…

[wyattpeak]

Oh, excuse me, I entirely missed that section of the page. Makes sense there's a geographical distinction, I'm not American so I suppose I don't have the intuitive issue with it, but on reading I see the logic.