I also almost always only unclip one foot (I do the right foot). The only time I regularly unclip both is when I am planning to get off, such as at an intersection where I have to get up on the sidewalk to push the walk button in order to get the light to change. I often have to do that on my way home. It's really busy one way, but the light in the other direction can be a long wait, as traffic is light on that side.

I've had 2 falls. One due to a panic stop where I had a car pull right in front of me. I avoided impact, but was not thinking about unclipping until I realized I was stopped and falling over. The other time was when I clipped my right foot out in preparation for my step, but without thinking made a bit of a left turn before I stopped, which of course left me leaning in my clipped in direction. That was a "lesson learned" experience, and now if I think I may have to make a left lean, I will unclip both. Pretty rare, but after falling once because of it, I'm pretty good at remembering!