According to release notes of 1.9v, this point was mentioned in the improvements that:
"If customers were using reward points and they had more reward point value than the total order value, they were still required to select a payment method"
But when any order is placed PAYMENT METHOD is empty (not mentioned) in Administration> Order Details, PDF Invoice, E-mail notification etc which makes this improvement useless because how store owner will know what payment method is being used by the customer even if his reward points are more than subtotal.
This Bug was reported here: https://www.nopcommerce.com/boards/t/7492/nopcommerce-190-is-coming-in-14-days-beta-testers-needed.aspx?p=3#30742