I guess I'm an oddball, but I prefer different systems for different bikes, and keep two sets of biking shoes: one comfy "normal looking shoes" pair with SPD cleats for both my fixed and mountain bikes, which are comfy and easy to walk around in. And another, much stiffer carbon pair with look cleats for my "serious" road bike, which I never plan to walk around with (unless something goes really wrong with the bike... which hasn't happened, yet). I would hate having to either settle for SPD on my road bike, or try to walk around in huge look cleats when riding my in-town and mountain bikes. I'm really happy with both setups.

Since you seem to be in a similar situation, I'd say it's definitely worth the slight investment up front to get a second set of SPD shoes+cleats for your fixed bike -- they're soo much easier to walk around in. But, they're not quite as stable when pushing hard as look pedals, so leave those on your road bike.