If you are using a newish computer with vintage keyboards, it's likely that you'll be using an active USB to PS/2 converter, anyway. At least that's what I always do. With the overcurrent protections that are built into USB interfaces, I think the risk of something damaging your computer is pretty minimal.
The dangers, I think, are more biological. All the pathogens and who-knows-what that are lurking on keyboards kinda scares me. Don't hit thrift store keyboards with a black light. *Shudders.* People eat, sneeze, cough, do... stuff... bleh.
For me, Used keyboards get their keys removed, if possible, and the keys are dunked in isopropyl alcohol (IPA). If they're still visibly dirty, they go into an ultrasonic cleaner. All the surfaces I can reach are wiped down with IPA as well, and crevices I can't reach are sprayed with 99% Methanol (it evaporates much faster than IPA, but it is more expensive and harder to get). Then the assembly dries for an hour, just to be safe. Then stabilizers will have to be re-lubricated after disinfecting, but they usually would have needed it anyway.
Now I can be assured (somewhat- I'm not autoclaving these things) that only MY pathogens are on the keyboard. Bwahahahaha.
I haven't killed anything (except nasty pathogens!) yet with this protocol, but YMMV. I have heard of cosmetic damage to Filcos and HHKBs from using IPA, but I've never actually had a problem myself- possibly because I don't have any Filcos or HHKBs. The lesson: test your cleaning agent on the bottom first to make sure you won't damage anything that's normally visible.