I wouldn't use it as a way to prepay your meals either since you don't get change back when using the vouchers. For example, if you're getting snacks that total $5.50, and you pay for it using a $6 snack voucher, you don't get any change back, and good luck trying to get something else that only costs .50 cents at DL.
The meal plan sure as heck doesn't save you any money, and actually you will likely lose money if using the plan.
You're better off using plain old cash or a Disney Visa credit card to pay for your meals and snacks.