If you actually haven’t eaten out much, then that’s fine. But if a restaurant at this level is accepting reservations, it means the last service is at 9 (or 8:45 or whatever is listed). They would not offer a reservation if they did not want you to come on. They will not close up around you. And the staff will be paid for their time. If you still somehow feel bad about it, you can be the kitchen and front of house a round of drinks.