MEDFORD, ORE. — THE state of Oregon - the home of the initiative, petition, and recall - is launching another experiment in self-government.Gov. Barbara Roberts (D), elected less than a year ago, is asking Oregonians to join together in deciding what government services are most essential and how to pay for them. "It'll be like an old-fashioned barn raising," the governor says. Critics call it just another excuse to raise taxes. If the turnout at a half-dozen introductory meetings around the state - what Governor Roberts calls "A Conversation with Oregon is an accurate gauge, residents are keen to take part in the effort. The state is one of only five in the country that does not have a sales tax; it is near the top in per capita income and property taxes. In tandem with Oregon's rapid growth (especially in the numbers of schoolchildren and elderly retirees), the pressure on budgets and other sources of revenue has increased to the point where voters last year took matters into their own hands. They passed Proposition 5, an initiative patterned after California's property-tax revolt Prop. 13 of 1978. Oregon's Prop. 5 caps property taxes used to pay for non-education government services at $10 per $1,000 of assessed valuation. The tax rate for schools is initially capped at $15, but then drops to just $5 by 1995. The measure does not set new sources of revenue, however, and instead obligates state government to make up the differences for local public-education spending. This would increase the state's burden from $560 million in the current biennium (the state legislature meets every two years) to as much as $2.9 billion in the period beginning in 1995. And it would mean that without a large increase in income taxes, a 40 percent chunk of all state spending would be just for local schools. The Oregon Alliance for Progressive Policy, a coalition of public-service employees' unions and liberal groups, warns that the state would thus drop from 21st among all states in terms of social-service spending to 39th. Last year, Governor Roberts advocated a new sales tax for Oregon. Since then, she has come to realize that what she calls a "quick fix a reform package including a sales tax - inevitably would fail. Between 1933 and 1986, Oregon voters eight times rejected a sales tax, most recently by 78 percent. At six meetings last month the governor met with overflow crowds. Skeptics wanted to talk about cutting government programs or fighting efforts to protect the spotted owl at the expense of timber communities. But many others signed up as volunteers to interview at least 2,500 community leaders over the next month about government services and how to pay for them. In all, 5,300 attended the six meetings and 3,100 volunteered to take a direct role in the initial process. This will be followed by hundreds of community workshops involving at least 20,000 randomly selected registered voters. Some of these will be open to public discussion, and the governor herself will take part in at least 50 meetings through "Oregon ED-NET," the state's new telecommunications network of satellite, microwave, telephone, and cable television technologies. Next January and February, Roberts intends to involve at least 100,000 more Oregonians in a series of meetings at townhalls and worksites where she will report back on her initial findings. In the spring of 1992, the governor and the legislature in special session will develop a tax reform proposal to be presented to the voters in the summer. So far, both Democratic and Republican leaders have expressed support for the process. Still, others think it is a waste of time. Former Gov. Vic Atiyeh (R), whose tax reform proposals were twice rejected by voters, told the Portland Oregonian last week: "Just to go out and say abstractly, 'What do you want?' isn't going to work. Nobody's going to ask for more taxes." But Roberts is convinced that Oregonians are "fed up with an inequitable, ramshackle tax system. "The time has come," she says, "to reassert our pioneer tradition of imagination and persistence."