I have 170 tables in an app and a similar amount of controllers. 3 of those refer to the logged in user and in no place except the admin controller itself can you modify an object, or what an object refer to, in such a way where an administrator can access anything he or she shouldn't. Admins have different accounts not to restrict what they can do but to (automatically outside what can be modified by posting params to models) keep track of who has made what changes and to keep them for making incompatible changes at the same time.
So no, I'd rather not start whitelisting my models.
Yes, however it's also worth noting that Rails core has acknowledged the awkwardness of relying solely on model-level protection for vulnerabilities that should be nipped in the bud at the controller level. Rails 4 will include DHH's new strong_parameters gem that allows params to be filtered proactively on every controller. This will of course help prevent a much broader class of vulnerabilities than ActiveRecord bugs.
So no, I'd rather not start whitelisting my models.