The DJ will need to get into the reception hall or venue prior to the guests arriving to set up the equipment and do a sound check. If the DJ is not able to do this in advance you will have to deal with the DJ and his or her crew bringing in the equipment during the reception. Providing access to the room prior to the event is really the best option.
A DJ may charge a flat mileage charge if the reception is outside of a particular area. If you are hiring a DJ from one area and expecting them to work the reception in another, be sure to ask about a travel of mileage charge. It may be more cost effective to find a DJ closer to where you are holding the reception if the charge is high.