"Rick Steves is a travel agent?"
No, he and his company are not travel agents. In fact when you book a tour if you would like help with airfare and other arrangements, then the company will refer you to a travel agent they use. (Elizabeth something or other)
To the OP, The reason for booking direct with the airlines is if there is a problem (delay, cancellation) you deal with the airline. If you use a 3rd party online travel agency you have to go thru the agency to sort things out and finding an actual person to talk to is difficult. If you use a local travel agent you may be able to contact them and have them re-book for you.
To book open jaw which means flying IN to one city and OUT from another, you'll use the Multi-city tab on the airline website. You wouldn't book 2 separate one-way tickets. It's not necessarily more expensive flying open jaw but you'll have to check. If you get a cheaper fare in to a larger city, say in to Milan and out of Rome, you also have to figure in transportation costs, time and perhaps another night in a hotel in that larger city. It's best not to travel a long distance by train on the day you depart. You'd want to have the previous night in a nearby city.
Airfares to vary daily or sometimes it seems, hourly. When you see a fare you can live with AFTER you have done research you should grab it!
Where are you flying out of? Perhaps someone here has experience? I can help you if you are flying from North Idaho, hahaha, but I doubt that.
--->editing to add: I see you mentioned Northern VA, so I assume that means Dulles?
Also, if you live in one of the cities that has a Rick Steves meet up, you can go have coffee with them and get free advice that way as well!
--->editing to add: Here is a link to the last DC area meet-up notice. You might message them to see when the next one is planned.
https://community.ricksteves.com/travel-forum/travel-meetings/march-4th-next-washington-dc-travel-group-meetup