By virtue of collecting Macs, I get a lot of computers with stuff already on them.
In many cases, I've been sold/given the computer with the owner's full blessing of prowling the stuff left on the hard drive. Usually it's something like "I know you're a chemist-there's some good software on there like..." or something along those lines.
I've also bought/been given computers with officially unsupported OSs installed and have received a warning to not wipe the hard drive if I wanted to keep running that OS due to the amount of work required to get it working. There are also special cases-like late G4 OS 9 Macs-where getting a fully functional OS 9 install can be tricky and I generally won't wipe those. I do "depersonalize" them, something that's fairly easy to do in OS 9.
As a general habit, though, I usually wipe and install my preferred OS and set-up for the computer.
I have found some interesting things, though. I bought a black MacBook with a "dead" hard drive once. It would hang on the Apple when booting. After I had installed an SSD and set it up the way I like, I attempted to repair the drive using some other utilities. I actually was able to repair it to the point that the computer would boot. The computer had belonged to a college professor, and had student records including full SSNs(it's beyond me why the school used those for ID purposes rather than student IDs). As I'm in academia and serve as a part time lecturer, I know the FERPA laws and how serious of a violation this is. I started to let the seller know, but on second thought took the drive out and shot it. Rendering it physically unreadable was the quick and dirty solution to holding what I considered a "hot potato" and I also had no motivation to do anything extravagant(like a 7-pass zero) to save a dying 250gb SATA laptop drive.
Fortunately, I've never found child porn or really anything of the like. If I found child porn, I'd be torn between shooting the drive or turning it over to the police(with details of the seller). Honestly, I'd probably do the latter for several reasons. The only thing mildly indecent I've found on a computer were what I'd term fine art nudes.