When do I use sanded grout as opposed to non sanded grout?

The difference in the two basically comes down to shrinkage. Because the non sanded grout has no sand in it to hold it's form it will shrink considerably once it dries . This will leave small, shallow, shriveled up joints. A general rule of thumb is to only use non sanded grout if the joints are smaller than 1/16th" such as ceramic bisque stackable tiles such a 4"x4" wall tile and small mosaics.

Other wise use sanded grout. A general rule of thumb is once the grout is smeared let it sit until the surface of the tile hazes over and then quickly remove the excess grout on the tile surface. Once that is done , come back over everything while the grout is still a little workable and strike the joints to uniform thickness and smooth out the surface to make it scare free.
 
For more information, visit Custom Touch Interiors