This is just my own personal opinion, but I would use the term "pc safe" when I had a pet that I legitimately traded for on the PC, and wanted to find another user here on cK that also had a legit pet that could be safely traded between us. In other words, trading a pet that's already been around the PC for another pet that has been around the PC. I wanted to avoid trading a PC-traded UC with a pet from a compromised account.
I don't think that "pc safe" should add any value to a pet though, I agree that if I was not known on the PC, I would avoid "pc safe" at all costs. I think it's all down to just personal preference and the situation.
I don't know if any of that makes sense, I'm tired.