The princesses do change a bit, and the ones I have seen rotate through are Cinderella, Aurora, Belle, Snow White, Jasmine, and Ariel. Merida and Rapunzel were alternating outside the Royal Hall area, and Anna and Elsa are on their own in the Animation Building in DCA. Princess Tiana is usually around New Orleans Square or around the River. I have also seen Princesses and Mulan meeting right inside the turnstiles and near Snow White's wishing well, and in a hidden spot behind Dumbo, too! Jasmine also meets with Aladdin and Genie in Adventureland. There is also the Princess breakfast/lunch at Ariels Grotto in DCA where several princesses entertain at a character meal. There will be lots of opportunities to meet princesses, the trick is to catch them before a CM is standing at the end of the line, turning away more admirers!
That's why the Royal Hall is awesome, with a bit of privacy for your pictures, autographs, and interactions. The Anna and Elsa meet and greet is almost as good, but they tend to usher in a few groups at a time. At Royal Hall, we never felt rushed!
Luckily, most of the line for the Royal Hall is visable, so you know about how many excited parties are in front of you. We waited about 15 minutes, and the line was only one segment long, without doubling back. My friend went at park opening and waited a lot longer, so I think waiting and watching the line may be a better strategy. Also, maybe right after the theatre showing the Frozen show starts seating, so the interested crowds in the immediate area are absorbed there instead of your intended line!
Have fun!