Romney absolutely should not agree to show any more tax returns than he already has. For him to show more returns is just a stupid Dumycrat ploy to try to hang him with his own rope. Even if everything in his returns is completely beyond reproach, that wouldn't make one little bit of difference to the Dumycrats.
What I'm saying is that Romney should call their bluff and allow the IRS to merely confirm whether, or not he has complied with the law sufficiently to satisfy his legal obligations. There should be a price to pay for this disclosure though and that should be in the form of the loss of the bet--be it Harry, or Mitt.
I think if Romney proposed the wager in the right way, it would embarrass the hell out of Harry Reid. Offer to donate the proceeds of the wager to a deserving charity. And if Harry weasels out of it, Mitt should say fine and donate the million dollars anyway. The publicity would be worth many times the million bucks and would put Harry Reid out of business.