Hi PT,
Here in the US (and maybe in other areas too?) quarry tiles are often shipped with a wax coating on them, them steam cleaned after grouting. THis is done in commerical jobs, restaurants, etc.
What I've done with quarries is to use a "grout release". It's not the same thing as a sealer. Instead, it's a thick, yogurt-like substance that can be rolled onto the surface of the quarries and allowed to dry. It does a great job of keeping the grout was biting into the surface of the quarry tile.
You're doing the opposite from us-just getting your first quarry job, and for me it's been some time since a customer had quarries for us to install. Good luck.