The whole purpose of the allegation is to provide the left and the media with a feeding frenzy year by year, return by return, page by page, line by line etc. Mitt can afford the best tax advisors and Mitt may not have paid any income taxes at all as Reid alleges. Good for him. The Kennedys went for decades with essentially muni-bond interest in the millions as sole sources of income. Not even required to be reported on one's 1040 until rather recently.
We can speculate all day about this, but Harry and his cronies know something, and Romney's resistance rather demonstrates they know something.
You know, even if that parable about the "eye of a needle" was a play on words about a guy on a camel going through a Jerusalem gate known as the "needle", the camel Romney is on is still working on the first hump and there are three more of them to go ~ he's never going to make it.