I agree that when camping in Grizzly country, the bear boxes available in formal campgrounds are well worth it. Once you're past western Montana, however, that's not an issue any more.
One advantage of the ACA maps is that they list lots of free and legal places to camp that are not actual campgrounds, and thus would not show up in any Google search for campgrounds. E.g., the town park in Hebron, North Dakota or the Bicycle Bunkhouse in Dalbo, Minnesota (neither of which has bear worries). My experience is that the ACA maps pay for themselves many times over just in saving me money on camping. Of course, if you're not on an ACA route, they are of no value.
As I said before, reservations are not usually necessary, and most campgrounds will accept a bicycle tourist even if the are full. One of the problems I have with reservations is that I usually don't know where I'm going to end up at the end of today, let alone a few days from now. Another problem with reservations is that I occasionally find the place I was heading for to be a unacceptable (and I have very low standards) and moved on. But some people like to make reservations anyway for the peace of mind.