I believe you have to bill for everything done, but even if it's non-covered you shouldn't have to write it off if you've notified the patient that is non-covered. The exception would be if your contract with the insurance company says you can't charge for non-covered item.
I just got a letter today, in fact, from Cigna saying we can only charge the patient for co-pays, co-insurance, and deductibles UNLESS we notify the patient that certain things will not be covered.