If your flight is on time then you will be at the car rental by 19:45-20:00. Immigration for US citizens has never been an issue and 19:00 is a pretty quiet time at IAD. I don't think Dec 23 should make a big difference. In terms of volume, I think there is a lot more passengers leaving the DC area for Xmas.
I agree that IAD is an odd choice for Philly. EWR is closer.