Yes, theoretically. Not in practice, because a default list conveniently exists, and people assume that everyone else uses it too.

[...] Members can make their own trust lists, and I would wager that most users who have been around here for more than a few months (excluding the signature spammers who probably don't even know what trust is) do exactly that. I would be against scrapping default trust, as I think it is probably most useful for protecting newbies and junior members who have not yet decided who they would and would not trust themselves, and these junior users are the the ones who are most likely to be scammed. [...]