Is the mortar coverage under the tile you install really important? You bet. Do you just assume that you are getting sufficient coverage? I hope not.
As often as mortar coverage comes up in articles about tile installation, you'd think that the problem would go away. Unfortunately, it hasn’t, especially in exterior and shower areas, for natural stone tile, and even high profile commercial installations.
If you want your tile installation to function as it should, then don't assume you have sufficient mortar coverage. Check it. That's the best way to avoid costly callbacks and ensure a successful installation