I don't know. But I have had my 942 on its end since day one. Its been going almost a year without problems. It is in a convenient place and gets great air circulation which keeps it cool.
I think that as long as it doesn't get bumped or moved while in operation it should be fine.
Except for possible cooling issues, it should be no problem.
You don't want the hot side low - and because that's the power supply side, it's the heavy side. SO, you gotta make sure the top-heavy setup is stable.
You'd want the fan at the top. Just look at a regular mini tower PC. Simon is right, the hot side needs to be on top. The rising air at the hot side creates a convection vacuum that sucks cool air in at the bottom. Placing the hot side dowm would mean that the hot air stream would rise bathe the circuit board with heat.