I wouldn't ride them AT ALL without cleats. Even with platforms, regular shoes can slip off, and that can be dangerous. I find M520s very easy to slip off of with regular shoes. I HAVE ridden them without cleats, but I wouldn't choose to if I had the choice.

Personally I don't see that much (really, any at all) improvement between cleats and platforms with toe clips, so I eventually just gave up and put the toe clips back on. It's just more convenient. I tried 2-sided platform/SPD but it was a pain getting the right side up.