Have you monitored the current seat map to see how it looks in terms of open seating? I would keep an eye on it and if the location of your seats in C are important, you may need to pay. Keep in mind that at 30 hours prior to flight time, you can go online and select seats for free. Although one time when I tried at exactly 30 hours, they had already put the seat map under airport control and I was unable to select free seats, so that can happen.

Maybe monitor the seat map regularly and if it looks like seats that would be acceptable to you are filling up, you can pay, otherwise try to do it at exactly 30 hours prior to flight time. You can refresh the seat map and it will go from paid to free from one minute to the next, as long as the seats you want are still available.