don't forget the detour around Baltimore...no propane in Fort McHenry Tunnel on I-95. use I-695 and cross using the FSK bridge.
as for your question about the beltway and I-270...the earlier the better. The DMV (DC Metro Vicinity) is ALWAYS busy. the earlier you can roll through the better off you are. but you shouldn't have any troubles on a Saturday.
I am coming out of Western MD. when i am south bound i use I-81 to US 17 and pick up I-95 in Fredericksburg, VA. its a pretty drive and keeps you out of the worst of the traffic.