We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember David Peters
saying something like:

QUESTION: Is drilling between two tiles (they look about 20 years
old) so likely to chip, crack or otherwise damage them that I'd be
much better off drilling somewhere else on the tile?

Use a good masonry bit and go slow. I've drilled many holes in the
grouting and never had a problem. What works in your favour is the tiles
are well secured to the wall and all grouted in - they can't move in any
direction so the hole just gets drilled with no drama.
--

Dave