While the casino world focuses on fighting an IRS plan to track slot jackpots starting at $600, a little-noticed companion proposal would benefit gamblers, a tax expert says. The proposed change in Internal Revenue Service procedure would establish a “playing session” concept, which could allow players to report less gambling income on their tax return than what is listed on a W2-G the casino issues to jackpot winners and the IRS. “The IRS is actually doing something that’s taxpayer-friendly with the session method,” says Marissa Chien, co-author of “Tax Help for Gamblers” (Huntington Press) and president of Advantage Tax Plus…