@sweetbabyjamie has twitted a great response to this. Basically a ladies virginity or cleanliness doesn't reset with the sun/moon.
A lady knows not to leave her last client's DNA for you to sample. And if she doesn't, do you think she cleans up for her first client?
Think of it this way... If you were a construction worker, would you shower up before heading out to work? Or would prefer to wash off the grime and filth of the day?
My advice is to select a lady wisely. And then don't over think it.