In Crawford v. Washington, I'm inclined to think that Stevens assigned the opinion just because there's such a massive difference between Scalia and Rehnquist's reasoning to reach the result. Rehnquist's entire concurrence is a criticism of the majority opinion; Scalia kills Ohio v. Roberts, while Rehnquist claims that there is no need to do so. Presumably there was some discussion before the show of hands about what the justices' understanding of the precedent was, and at such a time the division between Rehnquist & O'Connor and Scalia & the other six would be apparent. In a case where the concurrence sprang from a less fundamental disagreement, I'd think it more likely that Rehnquist assigned the majority opinion, read it and thought, "Eh, not exactly what I was thinking," found that it was what four or more others had been thinking, and wrote his own concurrence.