Of the two title fights, I think HHH and Orton is the one garnering the most publicity and air time, and remember it will have McMahon's involved in it, which is also likely to push it towards the end of the show. In addition, they have said approximately 9 billion times on Raw, Raw recaps, and all other media that Orton has a guaranteed Main Event match at WrestleMania. It would seem odd to say this if they end up going on before Cena.
However, in terms of the last match, if it is something truly huge, then it is worthy of following the title matches. Hogan v HBK at Summerslam 2005 is an example of such a thing.
Therefore the last match on the card, in my opinion, depends on who Jericho faces. If he has a 4 minute match with Mickey Rourke, put it in the mid card. If he has a match with a fairly regularly returning legend like Piper or Lawler, put it in the mid card.
However, if he is facing Austin, in Austin's last ever match, in Texas, then there can be no doubt in my opinion that this deserves to close the show. Austin's final farewell to the crowd would be a superb way to end the show, and nobody could really complain that it is devaluing anything if the man that is responsible for the majority of the success in the last 10 years for WWE, and wrestling as a whole, is given the best position on the card.