If it's a standard Portland grout you should seal it. Kinda like waxing your car, you don't have to, but you should. I'm a little surprised some tile people told you not to....or, did you go to one of the big box stores? You did use regular grout right? Not epoxy or pre-mixed stuff?
TILE GUY - retired- TROY, MI - Method & Product suitability consulting.
MARBLE from ITALY & GREECE -PERLATO-PERLATINO-BOTTICINO-THASSOS-SIVEC-VOLAKAS-CALACATTA- NAXOS-Slabs-Tiles-Custom containers shipped to you - MARMO.SICILIA@yahoo.com