I used epoxy on my wall marble listello that was just fine. There is a slight shadow that shows with epoxy, as it soaks in a tiny bit into the marble. I don't mind the look. I have heard that Laticrete makes such a finely graded particle in their epoxy that is is not likely to scratch the marble. It was fine with my marble.

This is a section of my listello with Laticrete's Silver Shadow. I post this so you can see the shadow of the epoxy showing through the slightly transparent marble - barely noticeable:
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I'm not a big fan of epoxy in residential applications, unless either A you have animals prone to accidents, in which case flooring should require it, or B if your tiling a countertop, in which case you don't want meat "juices" getting into the grout. Other than that, the benefit just doesn't justify the added cost.

Thanks to you both. Bill, do you find that grout tends to dry lighter than the sample sticks or darker?

Not the grout I was about to recommend. First of all, it's the latest generation of modified cement grouts. The color doesn't hardly vary from the sample stick. It's got anti microbial agents in it to keep mold and mildew from growing on it, and lastly, it's mixed with a very fine sand, so it can be used both in places where sanded OR unsanded grouts should normally be used. Also extremely stain resistant. It's Laticrete's Permacolor grout. Good stuff.