As far as it's ability to get dirty, I've never seen a difference between "sealed" and un-sealed grout. Like said above, a professional grout cleaning service, or some home brewed cleaners and elbow grease. For me, hot water and ammonia works wonders cleaning the grout on our tiled kitchen floor.
I've had tile guys tell me to never seal grout, be it on shower walls or floors. Just soak it and scrub it well when dirty. I'm sure as with anything, you'll get an opinion from every tile guy.