wrong. if you watch the deleted scenes in the trilogy release with the commentary on, zemeckis explains why old biff collapses after stepping out of the delorean, a scene which confused many audience members. if you remember in part 1, when it looks like marty is just about to fail getting his parents together, he collapses and his arm begins to disappear, because he no longer exists in his time line in the future. essentially the same thing is happening to old biff. by giving himself the sports almanac in the past and thus changing events that lead into the future, old biff is more or less erasing himself and creating a new time line.
anyway, keeping in tune with the thread, one of the scenes in back to the future (in front of the twin pines mall) was filmed in my neighborhood.