Ditto that. There seems to be some folk lore floating around about that type of thing, but I've never had a problem with washed yeast, and I haven't heard of any actual problems with it. Viability will drop over time. Some say use it in one week, some say two. Mr. Matly says the viability drops 1.8% per day. Going to do some cell counts next weekend to see how my yeast are doing and write a blog post about it.