WASHINGTON (Reuters) - Producer prices recorded their biggest drop in 10 months in March as the cost of gasoline tumbled, according to a government report on Friday that supported the case for the Federal Reserve to maintain its very accommodative monetary policy. The Labor Department said its seasonally adjusted producer price index fell 0.6 percent last month, the largest drop since May, after increasing 0.7 percent in February. Economists polled by Reuters had expected prices received by the nation's farms, factories and refineries to fall only 0.2 percent. ...