I don't think they are obligated by any means but they see it as an ideal matchup for "traditional reasons"
I think the rule is that if either the Pac-10 or Big-10 conference winner is tied into the NC game, then and only then, can the Rose Bowl select whomever they want. Last year when the Rose took Illinois, they didn't have to since OSU made the NC game, but elected to do so on their own for 'tradition' like you said.
What Conan said above seems to support this as well. I know for a fact that the exact Rose Bowl guidelines were edited a few years back, I just didn't know to what extent. That committee will be shitting themselves once again though if their hands end-up being tied to a PSU-OSU rematch.