Census Bureau reports may seem as dry as dust, but some of them provide crucial statistics for congressional decisions on taxation and benefit programs for the poor. Two of the agency's reports, one on after-tax income and the other on poverty, recently have been the focus of disputes between House Democrats and the Reagan administration.

Last month, Reps. William H. Gray III (D-Pa.) and David R. Obey (D-Wis.) released an analysis of a census report that showed that from 1980 to 1983 the top fifth of U.S. households increased their share of after-tax income from 40.6 percent of total income to 42 percent, a shift of about $25 billion from lower-income households to upper-income families.

But the two members complained that the president's budget did not include the funds needed to continue publishing the report, which shows how much money Americans have left to spend after they pay their income, Social Security and property taxes. They asked Commerce Secretary Malcolm Baldrige to try to find funds for the survey next year.

On a related subject, the House report on the Commerce Department's fiscal 1986 appropriations bill instructs the Census Bureau to cease "research and valuation efforts" that permit it to publish supplemental poverty reports that include the effect of in-kind income.

The government's official poverty statistics are based on cash income only, and last year showed that 15.2 percent of the population was below the official poverty line. However, the supplemental reports that Census released the same day showed that if the value of in-kind benefits such as food stamps, subsidized housing, government medical care and the like were counted, the part of the population living in poverty dropped to between 10.2 and 13 percent, depending on how in-kind benefits are valued.

Many Democratic politicians think that the Reagan administration wants to use the in-kind reports to justify cuts in domestic programs by arguing that poverty is not really as high as the official figures indicate.

Aides to Rep. Robert Garcia (D-N.Y.), who requested the language in the House report, said Garcia does not want to abolish the in-kind reports, just block them until methodological problems are solved and a way can be found to publish them in a scholarly context. The Senate has yet to act on the Garcia language.