For the Dulles part,
Unless you traveling with only carry on, you can't go to the main terminal and use the pre-check. Unless you don't mind the huge hassle of them bringing your luggage from the Midfield to the Main Terminal. I don't suggest that even to my worst enemies.
I have not used pre-check yet but I would imagine during the peak hours for Pre-Check would be bad. Think about it, 15-20 International Flights on United, plus the domestic United flights, American and Delta flights, there certainly are a lot of people. I do know that after 8 pm, the airport is a lot quieter so the Pre-check line is shorter(closes at 9 pm or so).
I don't know about during the week but on Saturday and Sundays from 6-9 pm, there are a lot less people leaving the Transfer point at C7 than during the rush hour 2-5 pm. Sometimes it takes 20-30 minutes before I see someone come up the escalator from FIS. Maybe fly into IAD from 6-9 pm or 10-12 pm on Star Alliance flights to not really deal with the long security lines at FIS.
It is so frustrating that A/B Concourse at IAD is so nice and C/D is awful. You had United spend money for the Regional Area for IAH, still at SFO(not sure if it is Regional area or the D/I area), new hangar at IAD but still nowhere close in deciding to renovate the gates etc at C/D.