Yes, 'I don't usually' would be the more common version, and certainly the one you would find in a textbook for learners of English. For this and other adverbs of frequency (often, sometimes, always etc. ), the rule is that it comes before the verb, or between the main and auxiliary verbs, or after the auxiliary verb in a short answer like the one you have used However, hearing the other in spoken English is also common. It's also quite common to hear it at the beginning or end of a sentence. Hope that helps!