Pluses to sealing: keeps it cleaner, less yellowing of the grout.
Minuses: probably voids your tile warranty (check); has to be redone every few years; doesn't keep out all dirt (kitty
can soak through pretty easily).
That being said, I'd say go for it. BTW, to clean the grout, I've found the Clorox bleach pens do a marvelous job. Tedious, but works like a charm.