We've done DCA first a time or two, but not much, we usually hit DL first and then hop back and forth. During our whole trip we go back and forth, I don't understand those who say they've been to DCA, that just boggles my mind. Our family enjoys DCA very very very much, I can't imagine going to DL only, I'd miss a lot of my favorite rides.
But yes, we generally start at DL and then hop over to DCA later in the day.