I wouldn't have thought it was a problem.
Two things you might want to consider are:
Are there any heavy components (transformers) that might put strain on their mounts? (Assuming its a switched-mode supply I wouldn't think the transformer is very heavy.)
Thermal convection. Even if there are no vent holes in the side, it might be designed for hot air to rise and go out vents in the back and if you mount it on its side the warm air might collect somewhere else. Again, since this isn't a high power device except perhaps the output amplifier, it probably won't be a big issue.
And of course all the electrons might fall out...