I got tiles, premix thinset, "blue board" water resistant sheet rock
in preparation to replace a section of tiles in my shower stall. The
guy in that dept where I got the stuff at Home Depot insisted I don't
need spacers for the tile, that shower stall wall tiles are supposed
to be butted up against each other.

All the "how to" videos I see on YouTube show the use of spacers for
setting shower stall wall tiles. You think this guy just didn't know
what he was talking about?