This is definitely true. If you lose your last game, you have a 100% chance of not winning the Superbowl.
Oh, you meant regular-season only....
But it makes sense that that would be true. I believe I heard yesterday that either the #1 or #2 seed in the NFC has made it to the Superbowl in every year since 1990. And in the AFC there have been a couple of #4 seeds to make it, but no #3's, 5's or 6's. It would be hard to finish with one of the two best conference records by ending the season on a two game losing streak if you think about it. You'd have to have a record of at the very minimum 12-2 before that...not many teams are in that position. With so many 1's and 2's going to the SB...stands to reason this would be true.
My records only go back to '99, but of the recent SB champs...
In '02 - Tampa Bay lost in Week 16, but won in Week 17
In '01 - New England ended the regular season by winning 6 in a row.
In '00 - Baltimore ended the regular season by winning 7 in a row.
In '99 - St Louis lost in Week 17 (mailed it in against the 5-11 Eagles), but won in Week 16.
Balt '00 (won) and Tenn '99 (lost) were both AFC #4 seeds. Balt was not your typical #4 seed though. 12-4 record. At the time there were only 3 divisions, and they just happened to be in the same division as 13-3 Tennessee, so got seeded 4th. Balt actually had the 2nd-best record in the AFC that year.
Hope that helps.
