Sanded versus unsanded grout for shower install

I was told by a Home Depot rep to use only unsanded grout when working on areas that are going to get wet like in a shower install. He said the sanded would eventually come out with water constantly running on it. Is this true? Is there something I can put on it to keep this from happening?

No, it's not true, that's a good one.

Let's start by telling us what type of tile and joint size your using.

There is only one reason for unsanded grout, and always has been, and that's a joint size too small for the grains of sand to fit into it. Grout originally was sand and portland cement. period. When the craftsmen of ancient Europe started using the tight joints you normally associate with natural stone today, they realised that the grout was only sitting on top of the joint because of the sand, and as a result they started using the straight cement as a grout so as to fill the joints completely, and that is still the case today. Any portland cement product shrinks as it dries, and the more the cement content, the more it shrinks, which is the reason for the sand. Then, the cement is nothing more than just that-- it cements the grains of sand together. However, in a tight joint, there's not enough material to shrink to the point where there are any negative effects. However, if you were to use the unsanded grout in a larger joint (1/8" or bigger), then you'd see the grout appear to sink into the joint, as well as develope shrinkage cracks. Not a good thing.