if there's no interest accruing i'd imagine there would only be fringe reasons to pay now, such as not having a different place (such as a bank) to hold money.
also side note, keep in mind after 20 years of keeping current with the student loans, they can be discharged.
another side note, IBR payments are probably the best in general, unless you like 'forcing' yourself into larger payments earlier in order to keep the accrued interest low. but you can make early payments on IBR too so yeah I don't understand why people don't do IBR. the interest rate on each plan is the same afaik