Hey John,

I can't speak for every manufacturer out there, but I can tell you the reason that lead to me including that on the instructions for my products.

It seems that with a mild shampoo, the oils from the scalp and any type of removal chemicals "neutralize" the shampoo to a certain extent. Using a shampoo that protects the hair, the first wash will remove the majority of the oils, and the second wash will cleanse the hair.

Also, I like to wash twice especially when the unit is off my head. This allows the base to get cleaned twice, which helps it to adhere better.

I actually spoke to the rep for the lab that I work with, and she said "I always buy high-end shampoos, and almost all of them recommend washing twice".

In the end though it comes down to personal preference. The instructions on the bottle are a guideline. Try different things and see what works best for you.