I tend to prefer did +not+use to. I feel it's more contemporary, as used+not+to seems to be somewhat more archaic/formal to me. I would use "Did you use to...?" in questions as well as opposed to "Used you to..?". Having said that, usedn't is widely used in Hiberno-English in its contracted form.
I've included this link used to - definition of used to by Macmillan Dictionary.
I think it gives a very accurate explanation of how this form has evolved in time.
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