I can't think of any good reason to actually start paying them off now, assuming these are subsidized loans and not just payment-deferred. If, however, payment is deferred but you're still accumulating interest, then it wouldn't be a bad idea to start paying them down.
But if the former was true, just pay whatever you would have paid to them into a high-interest U.S. savings account (like Ally Bank, HSBC, ING Direct, etc.) or even start a couple CD accounts when you've accumulated enough for that to make sense. You definitely shouldn't be using stocks or other non-guaranteed investments for such a short repayment time (<6 years).
The other advantage to saving the money now is that its still available to you in case you have an emergency and need it back.