I used to live in Rockville for about 25 years, DCA by Metro is generally your best option. In morning traffic, if you're driving, I would probably go with BWI, anytime other then that IAD is usually a better bet then BWI. If you allow yourself time to get to the airport you really have pretty easy access to all 3 WAS airports, which is one of the good things about being a DC based FTer living in Montgomery County.