^what he said. I was always told this by the mix engineer at the studio I used to intern at: As 'cool' as it may look, if their not meant to be on their side then dont do it. It can mess up the stereo image and that will screw with your monitoring.
I've heard depending on the monitors you don't want to lay em on the side. I would never lay mine on the side because I don't feel like they were made to be.
This actually hurts you in most situations. Tweeters are designed for a certain direction. Most monitors are not designed to do this for accurate stereo spread.