I have never done any stain testing. But I do simple single transfer monochrome work. If I have a light 'stain' around my print, I wipe it off with a wet cloth (carefully!) and otherwise don't worry about it.
Edited to add: The pigments I have used are several brands of lampblack watercolor pigments in tubes (also some ivory black and burnt sienna.) Some pigments do stain -- folks who do a lot of gum printing can point you to a list of pigments that stain online...bound to be one.