A lot of interesting and critical replies:
My question arose from a recent experience with some relatively ancient, but otherwise still very fine Champ decals whose decal paper simply would not "wet" in any kind of timely fashion, forcing me to eventually "push" the decal off- hazarding the decal in the process (the paper was still almost dry underneath the film). Also, increasingly, I am placing my decal water in very shallow jar lids, etc. with the water only about 1/8"-3/16" deep. In this circumstance, of course the effects of surface tension are closer at hand. I use the shallow pan/dish so that tiny decals do not sink out of sight when I am futilely chasing them around the surface (surface tension effect), and there is much less water through which I must futilely fish around to at last find the little @#$%^&* (if I find it at all) when they sink.
I have always used tap water and never, ever had residue problems, but using distilled water would also also seem a no-brainer (is there an expected difference in surface tension between distilled water -no minerals-, and tap water -varying amounts and kinds of minerals?.
I like the PhotoFlo idea. I would expect one bottle to last both me, my children, and my grandchildren at the rate I would use it, however.
Denny S. Anspach MD