About a month ago, Kiyota Jirokunietsu San came to supper at our aikido/Iaido dojo with his student Pierre Nadeau San. He was visiting in Montreal and giving a demonstration on nihonto forging.
http://www.soulsmithing.com/PERSONS/KIY ... otaEN.html
I asked him that very same question during the meal.
Kiyota San told me that he only use ushiko powder of good quality to clean his new forged blades. His teacher, Kawari Kunihira San, did the same.
http://www.mugenkan.com
For the first 3 month, the blade has to be cleaned every week because it can rust easely. After that it is cleaned once a month for 3-6 months and then only every 6 months. It has to be cleaned every time you use it or show it to someone. It has to be cleaned every month for 3-6 months if you move it out of its usual resting place (you move to a new house where the humidity level is different).
Rice paper, white flanel or special clothes for eyeglasses can be use with shoji oil (of good quality) to clean the blade.
Kiyota San does not know exactely why (there is seldom WHY with Japanese people), he learned it his way.
Hope this will help.
BenoitD