I wear my New Balance walking shoes; nice flat outsole with tough rubber, so the screw heads don't damage them much. Just as importantly, the tops are well ventilated to help me stay cool in the summer months.

I'm sure there's some benefit of having rigid soles, but with proper foot placement on the pedals, so that the toes and balls of the feet are on the pedal, I don't think it's a big concern unless you're racing. (in which case you'd be clipped in) Plus, with rigid soles, they're not really walkable any more. If you're going to go that far, you might as well clip in.