About only clipping out one foot, I personally think it is a bad idea (from my experience) since sometimes you may end up suddenly changing to the other foot or losing your balance. My first fall actually happened because of this--I was in a parking lot (don't do this, use a grassy field) practicing clipping in, and I started slowing down to try unclipping. And I had already planned out exactly which foot should come down, etc., so I only unclipped my left foot. But whaddya know, somehow my center of mass shifted to my right and I ended up falling over, not being able to do a thing since I couldn't unclip my right foot fast enough--scratched up the derailler on my nice new 04 fuji roubaix pro.

Lesson? Clip both feet out when coming to a stop, at least if you're gonna be riding in a busy place with cars and etc. Just in case you do something stupid like I did. And like Stubacca said, unclipping near kids or passing ain't a bad idea either.

Anyhow, usually after that first fall you'll never fall again.