A consumer complaint is ricocheting around the world: low interest rates are eating away at savings. Bill Taren, a retiree near Orlando, Fla., discovered in August that his credit union would pay only 0.4 percent annual interest on his saving account, even though inflation averaged 2.8 percent over the last year. So he and his wife decided to just stuff their money in the mattress, he says, because at least there “we can see the cash when we want.”