Why does it matter what a given candidate for political office pays in taxes as long as what was paid stands up to an IRS audit? It is highly unlikely the candidate had anything to do with writing the current/past tax code.
How many crazies are there out there (and there are a few) that would say: Oh I only had to pay under the current tax laws 15% and I feel so guilty here's another 15% just for the heck of it?
