The first long trip I took on a motorcycle (sometime around 2004 or 5) I just headed out and rode. I did not have a GPS then but did have a map in the bike's (Valkyrie) saddlebag. I then got a GPS but the Magellan did not do routes. Then sometimes around 2006 I got a Garmin (6830) I think and stated planning trips.
I used Microsoft Streets and Trips to plan my trips (and yes I still use it today even thought the maps are out dated). I plan the trip based on where I want to go and what I want to see on the way there or when I get there.
Next the time available for the trip determines what roads I can and cannot take. A trip from SC to AZ and then North to MT with only two weeks to do the trips means I take the interstate to AZ and then non interstate roads until I turn East again for home. That sets the interstate miles to 500 or so and non-interstate at maybe 250 miles per day.
Today, being retired, I limit my travels to non-interstate roads as much as possible with daily mileage between 200 - 300 miles. I do know where I am going to stop and I do plan the routes to get there.
There are those who will say that is way to "controlled". I want to be more free in my riding, not prescribed.
I can only offer the following analysis. I have discovered, been on, many roads I might never have traveled if the route I developed did not take me there. While the "waypoints" were planned, the roads were not.
As I said, I use Microsoft Streets and Trips. I plan the whole route (current one is eight days) and provide that route to my family so they will know where I/we (when the wife goes with me) are going.
Next, I break the route into individual days and save each day as a GPX file. I then, upload that GPX file to my GPS (Garmin 665). I then let the Garmin device calculate the route and then I look at it on the device and see what it looks like.
I carry my laptop with me on the road so, if I need to, I can easily change the route using the laptop and the process above. I don't do it often but, on a trip to Florida some years ago, we did stop at a local Honda shop made some route changes (on the outside picnic table) based on what we were experiencing (lots of traffic and stuff). Took about 15 minutes to change the route and up load.
As for roads I want to take, I do use the website below to look at recommendations. It is not a great reference but it does provide some information I would not easily get.
The Best Motorcycling Roads logged by 45898 motorcycle travellers for bike riding
www.bestbikingroads.com
As I said, I do plan the end point. I am old enough now that I really want to have a reasonable place to stop on my trips. And when the wife is along, its even more important.