I have seen several on their sides before... including some or the good old HES rigs. If I were to do it this way I would be certain to lay it on its MOBO side DOWN so that gravity and/or transport etc trying to remove any pci cards you may have installed etc etc... as well as minor things like large cpu sinks haha. See the attachment for how we do ours: Mac on the left, 2 23" monitors in the 2 slots to the right, 5ru for LED processors, DAs, etc, misc discs etc in small pull out, and a UPS in the bottom.