There is a wide variation in what folks prefer. Some dig out the traffic count maps and find roads with very light traffic. I am more inclined to a bit larger more traveled roads.

Improvising a route is fine, but you can't go too far wrong with the Adventure Cycling routes either.

Personally I like the type of roads AC selects, I like the fact that I will probably meet other touring cyclists once in a while, and I like the extra info on their maps. It is really handy to have contact info for services including free places to stay, campgrounds, bike shops, libraries, hospitals, post offices, etc. They pick small enough and relatively safe roads and they avoid most cities in favor of small towns. They tend to seek out good scenery as well.

If you like to plan in detail, planning your route might be something you like. I personally don't care for a lot of detailed route planning in advance.

Improvising as you go can be fun too.

My personal favorite way to tour is to use AC maps where they make sense and improvise where I want to detour off of the route for whatever reason. If an AC route doesn't go where I want to go, then I like a loose plan combined with improvising as I go.

Reading some journals on the crazyguyonabike site might give you a bit more feel for what the various approaches are like.