Could be true. But, let us consider a scenario where that man did not wash his butt with soap and water (wiped only with tissue), so the butt remains smelly. However, after that he did wash his hands with soap & water. In this scenario, this man who may go to bed, may wake up with an itchy butt, but not with any smelly finger, assuming the soap with which he washed his hands is a good enough soap to remove the smell. So, in other words, conditions apply (both to the proverb as well as washing butt with soap and water).
EDITED later to address
@prey24 : And it is good when a weekend type of thread enlightens and sometimes educates us about some general life aspects, such as hygiene and sanitation or anything that relates to us staying healthy. As no matter if you are into internet marketing or any other work, staying healthy is the most important aspect of being productive. I wish everyone good health, always take care of your health, let that always be your first & most important goal apart from other goals.