I've been wondering about this as well, enough to have previously mentioned my astonishment that they'd spend *so much time* in Croatoan showing Dean picking up that bloody knife and, in Hunted, Dean scraping his finger against the sulfurous windowsills, etc.

Yeah, you have a point! I remember the first time I saw Dean lay fingerprints all around a crime scene (Nightmare maybe?) the camera lingered on his fingerprinting, and I exclaimed, "Dean! Fingerprints!" but then two minutes later, he specifically said to Sam to wipe down the fingerprints. So I could totally buy they are doing it on purpose.