When it comes to challenging the conventional wisdom, few businesses can match the chutzpah of the $ 4 billion-a-year indoor tanning industry. Faced with condemnation from health organizations and government agencies, trade groups representing the nation's 50,000 tanning salons are fighting back with a media campaign. “New research shows that moderate tanning prevents cancer,”one press release announces.   The industry faces a slight PR obstacle, however: No leading expert accepts such claims. They all warn consumers to stay away from tanning booths, saying that the ultraviolet rays from the sun―and tanning lamps ―can cause skin cancer. Scientists generally accept the view espoused by the American Academy of Dermatology: “There's no such thing as a safe tan.”   It's exactly that sort of downbeat message that prompted Joseph Levy, the executive director of the leading trade group, to start running monthly ads this year. “We're trying to undo years of totally negative conditioning by the medical community,” Levy says. But critics insist that the industry is trying to make its case by drawing on unproven theories and exaggerating current research. For instance, the group's ads and press releases highlight a study that “suggests that 30,000 cancer deaths could be avoided every year if more people tanned regularly.” The study was actually a review paper positing that sunlight exposure can prevent such deadly cancers as colon and breast cancer, because people who live in sunnier climates have lower rates of those cancers. But epidemiologists say that other key factors, such as diet and exercise, also differ in warmer and colder climates, so sunlight can hardly be proclaimed as a cure for cancer.   The incessant attacks by health organizations may be having an impact. A decade ago, indoor tanning was among the fastest-growing industries, but growth has slipped to less than 4 percent this year, compared with 54 percent between 1986 and 1988. Still, Levy is confident that the 21st century will be a “Bronze Age” for the tanning industry.