'TIS THE SEASON to be charitable, and flocks of worthy organizations are soliciting aid - often by mail. As staff writer Janis Johnson reported the other day, sophisticated direct-mail fund-raising may have produced as much as three-fourths of the nearly $30 billion that Americans donated to charities in fiscal 1977. Yet recipients of these appeals often know little about the organizations that seek their support. Most solicitations do not describe how the group is run or how its money is actually used - how much goes for actual charitable work, how much is consumed by administration and fund-raising costs and whether any is diverted to other activities.

A citizen who wants to check can often get this information from the charity itself. Over 200 philanthropic groups now meet the Council of Better Business Bureaus' standards for ethical fund-raising, truthful advertising and disclosure-on-request. Some material can also be obtained from regulatory agencies in some states or, regarding nonreligious groups, from the Internal Revenue Service's public files.Beyond the dictates of laws, more and more charities now recognize that disclosure not only helps people to give wisely, but also helps well-managed charities to avoid being hurt by the dubious doings of some others. In the wake of the scandals involving the Parllottine Fathers of Baltimore, Catholic charities have become far more willing to reveal financial data traditionally swathed in secrecy. In a major step last month, the National Conference of Catholic Bishops approved strict guidelines for disclosure and accountability for all church-affiliated fund-raising in the United States.

Some members of Congress and major secular charities think that disclosure should be required. They are backing a bill before the House Post Office Committee that would require most general appeals for mail donations - including broadcasts that ask people to send in funds - to state, among other things, what percentage of the group's income goes directly to its charitable work. The largest exemptions would be groups' appeals to their own members and schools' solicitations of alumni and the like.

Much as we support the principle of disclosure, we have real reservations about H.R. 41. It would inject the government into aspects of philanthropy that have traditionally been regarded as exempt from federal oversight. Although its sponsors have modified the postal Service's proposed power to set accounting and disclosure rules, the potential for intrusive regulation remains. Many religious groups are especially concerned - but even the general exemption that they seek would bring the government more into the business of defining what is a church and who is or is not a member.

This is not to say that congressional concern is valueless. The bill's existence and recent publicity have pushed many charities toward greater self-discipline and voluntary disclosures. The Billy Graham Evangelical Association, which has been under fire, released a detailed financial statement this fall. Last week representatives of about 30 evangelical Christian charities agreed to try to devise uniform financial standards. With such self-policing efforts under way, Congress ought to hold off on any bill. The ultimate discipline for charities, after all, is the decision of individuals about whether or not to give. Government ought to tread lightly in this philanthropic realm.