If you're not picky about when and where you want to dine, and you're happy with counter service if everything is booked, then no you don't have to make your arrangements now. But keep in mind that lots of places are already full at prime dining times, and many places will not be taking walk-ups as that's a busy time (and I think
free dining is offered then, too).
Norway is now a princess character meal, so not the buffet you probably enjoyed =(
I'd check out the menus at
www.allearsnet.com (under dining) and see which places you think your family would enjoy. If you want a character meal, then YES you need to make a reservation ASAP for it as they will probably be booked. If you want a popular place, like 'Ohana, CA Grill, Le Cellier, Sci Fi, 50's PT, etc then I think you should make one soon as they often don't take walk-ups. Also if you want a dinner show or Fantasmic dinner package.
So in answer to your question - yes and no! =) Remember that in most cases, ADR's are free to make without penalty if you cancel (or don't show), so it doesn't hurt a thing to make one to ensure you have a spot to dine.
Good luck!