Yeah, I've heard the First Evil thing, too. I've also heard that Nicky Brendon was given the option to be killed off (I think in S4), because the writers thought they'd run out of ideas for Xander's character, but Brendon turned them down, because a paycheck's a paycheck.

I never know if any of these rumors are true, though, lol. Never heard the "killing the cast in S5" one before this.