Well, rather than random I'd prefer hash-based, but random will have to suffice if that's not possible. I keep stuff in various folders (bikinis, nudes, underwear etc etc) for neatness, but I occasionally have a total overhaul of the system when it gets a bit messy (which it always does, I'm not always brilliant at keeping things in the right folders heh).
Trouble is, I often batch rename stuff as well, and sometimes end up with a system where tons of files have the same filenames, only in different folders. That makes things difficult when I come to re-organize again, so I want to rename everything using hashes so they're all guaranteed to be unique :)
Obviously stuff saved directly from imageboards isn't generally an issue since those are often hashed filenames anyway, but going through thousands and thousands of images manually to sort out those filenames that aren't a problem isn't something I feel inclined to do :p