I generate my ride maps using Microsoft's consumer mapping software; and you can easily export the waypoint coordinates to a GPS.
Is it common to use GPS for long-distance tour navigation? I can see many advantages, so long as the units are rugged and waterproof, and you've got spare batteries, and some minimal kind of paper map backup.
It would be great to get a signal to turn left, turn right, just before you need to turn. Then you could pick really obscure, poorly-marked little roads that represent the shortest or most scenic distance, and not worry so much about getting completely lost.