The Board of Selectmen adopted a reserve-fund policy Aug. 20, following the July 22 downgrade of the town’s bond rating by Moody’s Investors Service to Aa3 from Aa2. The policy says the town “shall endeavor” to maintain reserves of 5 to 10 percent of general-fund revenue, but it isn’t a mandate, because appropriations are the prerogative of Town Meeting. The policy was recommended by Finance Director William Fowler, who said in an interview that Sharon has a little more than $100,000 in reserves; 5 percent of revenue would require about $1.2 million. The town has depleted its reserves to hold down taxes in a weak economy, he said. Moody’s cited shrinking reserves as the primary factor in the downgrade. With a lower bond rating, the town will pay more to borrow money.