If a team wins, they shouldn't fall, or if they do, it shouldn't be by much. All teams (with the possible exception of Bama this year) have off games. If you punish them for pulling out a win that could leave them out of a national title hunt. That is what makes a good team great, they find ways to win when they aren't playing well.
Look at the 91-92 Bama team that won the national title. They beat LA Tech by a score of 13-0. They beat a bad Vandy team 25-8 and they just got by Southern Miss at 17-10. You could easily say that isn't the resume of a top 5 team, much less national champion, but it was.