In principle, there's not much reason to think that the construction isn't perfectly regular, and you will find the following used:
Didn't they use to come here often?
But phonetically, "use to" and "used to" are practically indistinguishable and almost certainly not reliably distinguishable. In practice, you will see the following more commonly written down:
Didn't they used to come here often?
In speech, it's not really clear what percentage of people are 'really' saying use and what percentage are saying used.
The following slightly archaic forms also exist:
Used they to help you?
Used they not to help you?
Usen't they to help you?
Of these, usen't they is also used to form tags (albeit not so commonly as other forms such as didn't they, wouldn't they etc).