I've read reports that guests successfully skip that field. When you check in at the front desk (without using on-line check in), you are not required to give them a credit card. If you don't, you just can't charge back to the room. So I don't see why they would require that field for on-line check in.
IMO, there's nothing to lose by trying to skip the credit card field when it's time to do the on-line process (10 days ahead of arrival). If it won't let you skip it, then just don't complete the on-line check in.
Online check in isn't going to save you any time at the front desk anyway, given that you want to use the rewards & gift cards.