Very. In order for a spell to be properly effective, the target's full Christian name must be used. Any attempt at using epithets or descriptors might end up changing the magic. I can attest to that myself: a spell cast on me and a spell I cast both had unintended results because the target wasn't properly named.

[ there's a brief moment where Strange thinks 'oh God, I sound like Norrell' and kind of hates himself for thinking so. ]

Likewise, I've felt...something when a true fae's used my full name before. Assuming we don't piss them off to the point of no return, I doubt any of the fae would try to cast a spell just using partial names or titles. But I don't want to take that chance.

[ especially since yeah! He's cursed! And has absolutely no goddamn way how the Prince would interpret that. ]