As the years pass by and you pull out the wedding album for a romantic moment by the fireplace, you will see wonderful memories that your photographer captured on film. If you have decided to make a live band the feature of your reception, I would advise booking the band first so you can better plan your reception location.
Most couples select the reception site based upon the number of guests, parking, food, etc.
So, most often the reception site is already chosen. However, providing the band with the optimum placement in the room will help them a great deal and will improve the sound quality, not to mention, the energy level. If the group is cramped, they can't move around and if the acoustics are bad, the sound may not be the best that they are capable of producing. You may have seen them in an optimum setting. It is difficult for a band to create a high-energy atmosphere when they have to be concerned about hitting one another in the head with a guitar neck or stumbling over monitor speakers.