The permanganate bleach can be less reliable since if the pH becomes alkaline manganese dioxide will precipitate. If this happens in a region of the film you will get a brown to black stain. The permanganate bleach can also cause softening of the emulsion. The dichromate bleach causes less problems and is safe if one is careful. Before disposing of the dichromate bleach add a bit of sodium sulfite to it. The color will change from orange to green showing that the chromium has been reduced to the less problematic Cr (III).