I would like to make another suggestion. While ACA maps are great for what they are designed to do, they do take some of the "adventure" out of the trip. We planned our own "Trans-Am" route, essentially following Highway 20 from Newport, OR to Boston, MA. Not knowing what was ahead, other than what we could glean from state maps and local information, was a big part of the adventure. We only saw about 6 other cross country cyclist on the whole trip (except where we crossed the Trans-Am in Yellowstone). I consisered this a plus, but others may like the social aspects found on the main routes. People treat you differently when cyclist are more of a novelty, than the norm. We even got interviewed by a small town newspaper!
On the other hand, we have also done the Pacific Coast route which does not offer any options. We talked to many other bikers, and found the trip enjoyable, but in a different sort of way. There are not very many "unknowns" on the coast route; numerous state parks, motels and more clam chowder than you want.
If you follow the advice from the folks above, and ask youself what you you want to accomlish on your venture you will be on the right track. Good luck, and regardless of the route you will not regret the trip.