Sounds sort of extreme. Perhaps they mean when the materials have been REALLY abused; like with peanut butter and jelly fingerprints? I don't think washing of (conventional) photo materials would cause a major problem. However I'd hesitate with very old photos with cracked emulsion. Maybe we have a museum expert somewhere? The "soapy water" was a clue. Most clear shampoos will work like a wetting agent (as with Photo Flow), so any drops film out and leave no water spots. [BTW, an old trick to print scratched negatives was to dip them in wetting agent and print them WET. Don't think I'd like to run this through my LS-1000....] I'd say, if your less extream methods (with well preserved materials) work, continue to be gentle with them. Get rid of grain? I like grain.