nuremberg to munich

We just returned home from Germany yesterday. We went to Neuschwanstein and Hohenschwangau castles from Munich. We reserved the castle tickets ahead of time from their website. We reserved Hohenschwangau at 1:15 and then got tickets for Neuschwanstein at 3:15. We took a train out of Munich at 8:52 (used Bayern Pass and single tickets to make up for the first part of the ride since Bayern Pass isn't valid till 9 am). We made a rather leisurely day of this, returning to our Munich hotel in time for dinner. There is lots of info, including details on how to take the train, on this site. Search Neuschwanstein and you can find the info that helped us plan our day (thanks, everyone!)

In short, except for on a very popular holiday, I would not book tickets in advance. The only train I have ever been on in Germany (I've been on about 200) where I didn't have a seat, was an ICE from FRA to Mannheim on the 2nd day of Christmas, a popular holiday. Although all seats were occupied, few were reserved. Had I made reservations, a few minute before, when I bought by ticket, I would have had a seat.

Local trains to Fuessen originate from Munich or Augsburg and take about two hours. You might have to make a transfer enroute, but if so, it's a walk-across-the-track affair in a really small town.

There is a Nuremburg-Fuessen local train that requires you to change in Kaufbeuren, so you could do that with the Bayern-ticket.

Don't bother with buying tickets in advance - you can't reserve seats on local trans anyway.

Munich is a better staging point for excursions to the castles. If you decide to arrange your own excursion, its much closer, and if you decide to take a tour, you'll probably find many more departing from Munich (if any exist at all from Nurnburg). And as others have said, you should have no problem buying tickets the day of travel.