Getting into magic spoiling territory here, so will tread carefully ..
Consider that there have been occasions when 2, rather than 3 princesses were in the Royal Hall, and the problem should be obvious. Also, the size of the Anna & Elsa location precludes what happens in the Royal Hall.
Back to the original question, it's better to see Anna & Elsa first thing on a day in which there is no early admission. If you can get in on an MM/EMH day, you can focus on rides for that first hour. So, you may consider going to the chalet on a morning when their meet 'n greet will probably start at official park opening.
Consider that there have been occasions when 2, rather than 3 princesses were in the Royal Hall, and the problem should be obvious. Also, the size of the Anna & Elsa location precludes what happens in the Royal Hall.
Back to the original question, it's better to see Anna & Elsa first thing on a day in which there is no early admission. If you can get in on an MM/EMH day, you can focus on rides for that first hour. So, you may consider going to the chalet on a morning when their meet 'n greet will probably start at official park opening.