I strongly support pre-planning and I find it fun to do but I also agree with the advice that it's not necessary to reserve hotels. If you find yourself battling bad conditions and behind the pace you need to make for the next hotel you could wind up making a poor decision based on "get-there-itis."
When I make a Christmas run I make myself a notecard with a list of the large towns along the way with a mileage between them figure so that I have some go/no-go information handy for making the decision whether to stop here for the night or go to the next town. The decision is based on how fresh I'm feeling and what the road conditions are (clear, dry, light or dark and starting to ice over after a day of battling snowstorms....).
Weeks beforehand I've also used Google to study what hotels are at each of the major towns so I have a sense of whether there are lots of options and where in town to find them.
And finally my wife, who is an excellent navigator and GPS operator, will use the GPS to callup the hotel choices and their distances as we're making a decision.
We've not had any issues with getting a room at the first place we try.