I very much doubt if putting on its side would be a problem as long as the ventilation is not restricted in any way.
"In the old days" PC system units used to sit "lengthways" on the desk under their screens. Then people got fed up with the space they took up (plus the fact the the screens were too high) and asked the question "would it be OK if I put the system unit on its side on the floor". Loads of people did and I never heard of a problem arising (other than that the units got kicked and sometimes screwed the hard disk - I have to own up to doing that myself). Back then the concerns were not only ventilation but also floppy drives and hard disc drives working in a different plane. But such worries have long since been forgotten.