I've sure harvested, washed, cooked, and eaten a lot of poke in my life, but never had to use a tub big enough to bathe in, so I never heard of it causing itching.
When I was doing gas leakage surveys in 1993 along the Pennsylvania/New York border, I found lots of people had poke in their flower beds around their houses as an ornamental plant. But in asking around, I never found anyone up there who had ever eaten any.