The more casual the place, no, you don’t need to make a reservation. If it is a higher-end place, reservations are required. In between, they are recommended. This is because, in part, many sushi-ya are very small so there is limited seating. The most important thing is that if you have dietary restrictions you must make a reservation ahead of time and notify them at the time of the booking. If you can’t ask your hotel to call for you, perhaps stop in the day before to make a booking for the following day.