I had ceramic tile installed 14 years ago, and have rubber-backed throw rugs at the back entry, kitchen sink, and a non-backed 8 x 10 in an eating area. There has been no discoloration whatsoever in the tile (quite inexpensive tile, I might add) or the grout due to the various rugs. That being said, you will find if the grout is a medium or light shade, it will over time become soiled where there is the most traffic or it isn't covered with a rug. It remains pristine where it's covered. I put two coats of sealer on my tile, which is what was recommended on the container. It worked pretty well over the years. However, I recently read where it's even better to put multiple coats of sealer on (they said ten).
Guess it couldn't hurt. Think I would go with five. Tedious but well worth it. My comments apply to sanded grout only, I've never dealt with the silicon grout which they say doesn't stain. I'd like to hear comments about silicon grout, as well.