Hi John,
I'm with you mate, viscose does lose its strength when wet (80% aparently)! Be carefull!
Especially on heavily soiled greasy areas such as arm rests and head cushions. If you want to take it on proceed with caution.
As extra agitation and work is required on the problem areas be very carefull. I've seen split fabric and ruined furniture due to over-wetting and over aggressive cleaning on high % viscose upholstery.
If its really soiled I would advise you to not clean it or advise customer of possible problems and get them to sign a disclaimer (stating that attempted cleaning could result in fibre degredation).
If they agree to your clean and sign disclaimer and are happy to go ahead, then go for it but treat problem areas with care.
Hope this helps,
Jason.