I would also be *very* wary of shady key sites, but you shouldn't worry about using the ones that aren't shadyNo, Steam does have an offline mode that works fairly well these days (that didn't use to be the case).
Also, I'd be *very* wary of buying keys from shady key sites. If you want to support Reiza, buy directly from Steam.
I would also be *very* wary of shady key sites, but you shouldn't worry about using the ones that aren't shady