I sealed my marble tile before installing it. I haven't used it since- did not seal the grout, but haven't had any problems with it. This is marble tile in the niche behind my stove.

This sealer is easy to use. But, I found most of my information by searching the bathroom forum where all the tile experts are.

On the back of our Carrera tile it says to put it on before grouting. I suppose the thinking is that the grout is kind of wet and it will protect it.

we don't have marble but our stone guy (5th generation) sealed after the backsplash went in. He put the granite in, then the backsplash (several weeks later b/c I had trouble deciding) then grouted, then sealed everything with 511 all in one fell swoop.

I used 511 porous plus (not impregnator) on our limestone floors, and will be using it on our marble countertops. From the FAQ on the website, it seemed like PP was better to resist oil stains, which I felt was important for kitchen counters. We sealed all the floor tiles with 2 coats before laying and grouting them. Then we will do another coat to protect the grout. Personally I would play it safe and seal the tiles before they get grouted, but really I don't know what would happen (if anything) if you didn't.

Thanks for sharing your experience everyone. Did a bunch more research, since the Miracle Seal people never got back to me, and decided to seal before grout. The main reason seems to be that the sealer will protect the marble from getting stained by the grout. I did the seal today, tomorrow grout, and then seal again.