short answer: NEVER

I'm not being trite. I assume you mean the personal income tax and I am an advocate of a Flat Tax.

Let me explain...

I believe that history bares out that what markets and businesses from Main Street to Wall Street loathe above all is uncertainty. Specifically uncertainty in economic policy from their government. I posed (near the end of our New Deal debate) the theory that perhaps FDR's death ended the economic woes of the Great Depression era more than anything else. His year to year new proposals, the previous one's needing annual renewal, guessing what portion of the private sector he might wade into next, business didn't know what to expext. I dub it a policy of "systematic uncertainty."

What is its' opposite? A policy of systematic certainty. And nothing is more certain than a flat income tax. And as a means of getting the camel's nose in the tent, I'd even compromise to a three tier flat tax: 5% for $1 above the Poverty Line to $50k (incomes below the Poverty Line pay zero); 10% for $50k to $250k; and 15% for those at $250k plus. No deductions. Your tax returns are essentialy done on one peice of paper. To that, if we're talking what to do right now, I'd add a cut in the corporate tax rate, from 28% to 15%; and a cut in the Capital Gains tax rate, from 15% to 5%. Implement that and its morning in America again, on steroids.

Of course, I'd get hammered for, "Raising taxes on the poor and lowering taxes on the rich!", so I'm only talking about what could be done if adults ran Washington... which isn't the case..