My experience with denied boarding (and I have quite a bit with voluntary) is that VDB is based almost solely on the first to volunteer. IDB is based on last-checked-in.
John
Perhaps this has been pointed out somewhere already but NW is testing in several select cities a "hidden bid" system for VDBs. In other words, you will set the least amount you will volunteer for and the lowest bidder will be selected. The days of the automatic $300 DBC may be gone (actually it was never supposed to be automatically $300 but I realize most agents don't offer less as they are instructed to).