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NEW YORK (CNNMoney.com) -- For most Americans, Wednesday is the day they have to make sure they're square with the tax man.

It's also a day that focuses the mind on those perennial questions about the fairness of the U.S. tax system. Are higher-income people paying enough - or would raising their taxes depress their incentive to work? Are the poor getting gouged - or do too many get off without paying a penny? Are those in the vast middle getting squeezed?

The questions are particularly relevant these days, as deficits pile up, demands for government spending soar and many of the past decade's tax cuts near their expiration in 2010.

Going forward, the White House has said it wants to cut taxes for families making less than $250,000, while at the same time start collecting more from families with bigger incomes in 2011.

Of course, the burden of funding the federal government isn't designed to be spread equally. The U.S. tax code is progressive, meaning that higher-income tax filers pay more in taxes than those lower down the income scale.

But just how much more?

The highest earners pay the lion's share of the dollars Uncle Sam collects.

The top fifth of households made 56% of pre-tax income in 2006 but paid 86% of all individual income tax revenue collected, according to the most recent data available from the Congressional Budget Office.

Narrowing in further: The top 1% of households, which made 19% of pre-tax income, paid 39% of all individual income taxes.

The trend is similar if you count income taxes, social insurance taxes, excise taxes and corporate income taxes (such as capital gains) combined. The top fifth of households paid 69% of all federal taxes. The top 1% paid 28%.

But researchers also note that the highest income taxpayers derive the most benefits from the tax cuts put in place since 2001.

Next year, as a result of all those tax cuts, filers making more than $1 million will enjoy a 7.7% average boost in their after-tax income relative to what they would have if the tax cuts weren't in place, according to the Tax Policy Center. Middle-income households, by comparison, will see an average boost of 2.6%.

A Tax Foundation survey found 56% of Americans think the amount of federal income tax they pay is too high. Those most likely to feel that way, according to the survey, include those making between $35,000 and $50,000.

But once the various tax breaks to which they're entitled are counted, the burdens of low- and middle-income tax filers as a group has been fairly low.

The Tax Policy Center estimates that for 2009, 43% of tax units (most of which are lower income households that may or may not file a return) will have no income tax liability or will have a negative income tax liability, meaning the government will actually pay them.

When measuring the tax burdens from income tax and payroll tax combined, the Tax Policy Center estimates nearly 12% of tax units will have zero or negative liability.

As for everyone else, it's likely their net income tax burden will not be high.

For example, in 2005, just under one in 10 filers owed more than 15% of their income in federal income tax, said Roberton Williams, senior fellow at the Tax Policy Center. Roughly 70% of them had incomes over $100,000.