In addition to Sheba's observations, I'd add that if you're really paranoid and you have the time, you can force your victim to take a shower before you leave. In addition to rinsing off any remaining trace evidence, you might enjoy scrubbing her down, and she might find it even more humiliating.
Personally, I take the opposite approach, and would likely have a short career as a rapist. My fantasies always involve leaving large amounts of DNA inside her.
But as our resident Frenchman, shysnale, might say,
vive le difference! 