I'm with the majority here. It's clearly the custom, so it's just something to get used to. (I, on the other hand, will always ask, "Okay if I resign?" first. About 75% of the time, I get no response within a minute, so I go ahead and resign at that point. The other 25% of the time; people say some version of, "Sure, go ahead." While it's fun to play our your killer engine, it's really not fun on the other end. And this is 1000x more so, when there are attacks in play, or where you need to respond during the future-winner's turn (e.g., Masquerade). I don't mind losing. I don't mind getting killed from time-to-time. But I really don't want to have to play a "dead" game for another 7 minutes, with each of her/his turns taking a full minute to cycle through.
So, count me as a vote for, "Good manners to ask first, or at least announce in advance. But not absolutely required." :-)