IMO unbound combos are ok for filtering, but if you present them as bound controls, expect that users will accidentally alter the field in records. If only looking at data, I'd open that form with the combo(s) locked and provide some other way of adding items to it. That might be a command button to add list items or perhaps dbl click event on the locked combo, either of which could open an add items form. Opening said form in data entry mode would be different.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.