Just about everyone on the Hill uses DCA when they can; they get free parking there and you can make the drive in just a few minutes (or take Metro from station near the House office buildings). Getting to either IAD or BWI is a pain in the rear that kills most of the time saved by taking a non-stop.
There's free parking at IAD for Congresscritters, too. Both Dulles and National are operated by the
Metropolitan Washington Airports Authority. BWI is not, and I don't think they offer free parking to Members but I can't say for sure one way or the other.