As far as I understand toning for albumen prints, the recommendation is to tone first before fixing. The traditional toners (gold-borax or gold-thiorea) basically leave the silver chloride undisturbed, while the high percentage of sodium thiosulfate in Kodak's Rapid Selenium Toner (about 30% as I recall) works to dissolve the silver compounds. Hence the very visible drop in density when the print is immersed in KRST prior to fixing. The fixer then removes the remaining non-image silver. So toning in KRST prior to fixing introduces two stages where hypo gets to work on the print.
Originally Posted by pelerin
I think that selenium toning after fixing gives more control over the final density and tonality of the image for two reasons: first, the overall image density is already established, and second, the dilution of the selenium toner, and the duration of immersion allows us to decide when the print "looks right." It's much harder to do this if selenium toning comes before fixing, since its difficult to visualize the full extent of the bleaching effect of hypo.