DMSO is a strong solvent and will carry anything with a molecular weight of less than a thousand through the skin. Thankfully before starting this experiment, I was already big into 'stone age' showering which for me is bathing using only water and scrubbing with a hand towel. Though I do make an exception and use soap underneath my arms and in my private areas. Also, if I am really filthy from working on my car or hiking, I will use soap and shampoo. However, whenever I use shampoo I will skip application of my solution.
My reason for limiting shampoo and soap is pretty basic. Really just a basic rule; If you wouldn't put it in your mouth then you shouldn't put it on your skin.
I am always as careful as I can be to limit cross contamination when using DMSO.