(not the topical anesthetic used by majority of dentists)
You should be very comfortable during the entire procidure, depending on the size of the filling and the amount of the work required to be done. You need an excellent job with minimal or no discomfort. If the filling is tiny and can be done without numbing, why not ? But if the filling can last for many more years, would still look like new, if both, you and the dentist are relaxed, by you being numb, you should be numb then. I hope this answered your question.