Some toners leave silver compounds in the emulsion, which can darken over time. This is especially the "bleach & redevelop" type toners. To avoid changes over time, the print should be fixed after toning.

If the bleach is KBr-containing, plain hypo is exellent: about 100g sodium thiosulfate (crystaline) to 1 liter of water. Add some sodium sulfite to prolong the life of the fixer to more than overnight. Use sodium bisulfite instead to get an acidic fixer. I use plain hypo for most of my post-process fixing, and for POP and van Dyke prints.

Any Iodine-bearing bleach or toner should probably be fixed in an ammonium thiosulfate fix. This however has a nasty habit of bleaching the image, so must be used with care. Long treatment in plain hypo is generally better.