That's ideal but if you're using cultured stone/eldorado whatever and your contract is just for a typical install there are always going to be 1/16"-1/8" gaps that if you use a light grey mortar someone can see it if they look close. Even if you're doing profit (ugg) the "stone" vary by 1/16" over a 16" piece, 2 of those high spots or low spots touching and there's a gap. If you use a dark coloured mortar that gap just looks like shadow
Just one of the tricks to trying to make thin stone look like real stone