I vaguely remember that at one time there was a rule of thumb that sorcerers should be trying to reach 70% crit because their heal depends on critical damage. Other than that, though, I haven't seen any guides or build discussions that specifically recommend crit thresholds for specific classes, and I haven't seen that one in a long time. Maybe it's because these days most build guides start with gear, and the gear choices have the crit thresholds already baked in. And I assume it differs between roles... tanks may not be as crit-focused as dps, for example.
But is there some kind of general rule of thumb? It seems if you're using the briarheart set, for example, you want to focus on building up your crit % in order to reliably proc the damage boost and heal. If you're using swamp raider, on the other hand, it's not necessarily as important as making sure you're using poison and disease abilities. But if you are trying to build toward a rotation where critical hits are reliable part of your damage, at what % does that happen? 50%? 60? Is it all changing next update, when crit goes down 6%?