You may shorten the life of the unit, as it was designed to be placed horizontally for heat dissipation, as has already been stated by others. But whether it will matter or not in this case is hard to say. The definitive answer will only be known by trying it out with several units, and compare with a control group of receivers that are run as intended. Since that is not practical, the question is, how lucky do you feel? I personally would place it horizontally on a shelf. If you lack shelf space, you might want to consider buying a taller shelf unit to accommodate all of the equipment you want to use.