I did some experimentation on this several years ago with an older Epson (probably a 1280), and found that it likes resolutions of 180, 240, or 360. Any of those come out very slightly better than my test photo's native resolution of about 215. The 360 & 240 were virtually indistinguishable to me, with 180 somewhat worse, and 215 slightly worse than that. I presume this is because the printer interpolates to some multiple related to those numbers, and its interpolation was worse than Photoshop's bicubic (which is what I used to get to 180, 240, 360, etc.).
The differences are extremely small, however. A casual viewer wouldn't have noticed. I had to get my nose right up to the prints and look very carefully to see any difference.
I've recently seen some people whose opinions I respect on another forum claim that the 180/240/360 advantage isn't true, and recommend letting the printer do all interpolation. I don't know whether they were only looking for larger differences than me, or if Epson's internal interpolation routines have improved in more recent printers. If the latter, then it's possible that my data are obsolete and someone should test this with a newer model printer. It's easy to test yourself - just take a small crop out of a photo with a lot of detail, use PS to upres or downres to various resolutions, print those and the original to the same size, and compare.
Lisa
