Note that I was not trying to print edge-to-edge borderless, but only that I got rid of the default Epson paper size borders, setting them all to zero.
The images I'm printing are all -smaller- than the paper size, by at least an inch, sometimes 2 inches. (In that case, even if the Epson default borders were left in place, it would not make any difference.)
The issue is that if I choose to center the image (using the 'bulls eye') the top and the bottom margins are the same (in PT) and the left and the right margins are also the same.
However when the image prints, the bottom margin is 4/10 of an inch larger than the top margin, as measured on the paper itself.
That said, I'll try again using the default paper margin size, and see if that makes any difference.