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.
I have NEVER made a mistake, I thought I did ONCE, but, I was WRONG!A+ BBB rating - est. 1987 - over 50 yrs. exp.