Screwing into the grout line between glass tiles
The question you want to ask is, how important is it for me NOT to crack any of the glass tiles? If I were doing it, I would say VERY important, as replacing the broken glass tiles would be a major hassle. I would recommend getting the diamond or abrasive drill bit regardless, and then using it even if you do put your screws in through the grout lines. Make sure your screws are smaller in diameter than these holes - you don't want to stress the glass tiles, whether through a grout line or directly through a tile. As a bonus, if you go ahead with the drill bit, you have the flexibility of putting the shelf anwy