To avoid mites, I only feed frozen. Plus it's safer for the reptile. Not saying you are endangering your animal, just in general F/T is more appropriate for the long run.
I agree with the post above as well. Buy them from a reputable breeder or one you know doesn't have a mite infestation within their animal population. The "treatment" for mites can be very harmful if done wrong or for that matter overdone. Better safe than sorry, I would find a better rat provider if you are that concerned.