Idealists say it is the world's greatest display of voluntarism; cynics call it the greatest collective effort at creative writing. Whichever, about 20 million American households will file federal income tax returns today, bringing the government's total revenues from the 1980 individual income tax to about $325 billion.

The Internal Revenue Service said yesterday that slightly more than a fifth of the 94 million returns it expects to receive this year had not yet been filed at the start of this week. If this year's pattern follows past form, almost all those returns will be mailed by the deadline at midnight tonight.

A few hundred thousand serious procrastinators will file Form 4868, asking for a 90-day extension of the filing deadline. The IRS always grants the extension, but the tardy are assessed interest and penalties.

As it has been for decades, the personal income tax again this year will be the government's largest single source of revenue. Personal taxes -- the income tax plus estate and gift taxes -- will account for about 45 percent of all the money the government raises. And as usual, members of the middle class will provide most of the money.

The IRS says that middle-income households -- those with gross income between $15,000 and $50,000 per year -- account for about 32 percent of the tax returns filed but pay about 60 percent of the income taxes. Even if President Reagan convinces Congress to give him precisely the tax changes he has asked for, these figures will not change much. The middle class will continue to be the government's chief form of financial support.

Studies suggest that Americans are among the most honest taxpayers in the world. International comparisons are somewhat difficult to make because of legal differences from country to country, but those analyses that have been attempted indicate that U.S. taxpayers are more forthcoming with their government than are Europeans, particularly in France and Italy, where tax evasion generally does not carry the evil connotation it has here.

The IRS, which generally reports extremely high levels of compliance with tax laws, says its last completed "Taxpayer Compliance" audit, in 1976, showed that people voluntarily pay 92.3 percent of the taxes that would be found due if all returns were audited. A 1965 study had put voluntary compliance at 94.3.

The General Accounting Office, on the other hand, has expressed concern in recent years about tax cheating. The GAO notes IRS estimates that $135 billion in income went unreported in 1976 -- "staggering estimates, yet they are probably understated," a GAO official told Congress last year.

The Reagan administration has particular reason to hope that taxes are honestly paid, because among the cuts if proposed in President Carter's 1982 budget was a $125 million reduction in law-enforcement and tax-auditing programs.

The administration's explanation is that there will not be a need for increased auditing if the president's proposed tax cut pases. The theory is that people cheat more when taxes are higher, so a tax cut would reduce cheating.

The Reagan budget proposes more money, though, for the IRS collection arm -- the people who try to make taxpayers hand over the money when they have been found owing. The IRS says this is the most lucrative use of its funds, because every dollar spent on collection brings in $5 in back taxes. It is also the IRS' most unpopular branch: "Collection is where they get the most assults," an official says.