I've used the HOG Ride planner, Google Maps, and Mapquest. They all seem to be pretty accurate with mileage. I usually plan around fuel stops and look to put on around 700 - 800KM (450 - 500 miles) a day. I use this as an overview so we have a rough idea of where we will be going as things often change with weather and construction.
I have maps of most of North America that stay in my saddle bag. These help me plan (or refresh my memory) for the next day, show others some great roads, and have settled many arguements. I like coming to a fork in the road and trying to remember which way to go so I don't use a GPS. Besides, getting lost makes for a better story...
I (and my wife) like to stop overnight at someplace that has a bed and hot shower, with decent food and a cool refreshing beverage within walking distance. Since we usually start by 7am this give us plenty of time to stop during the day if we want, or if the weather turns nasty. My wife isn't fond of riding after dark so we're usually done by 6 - 7pm.
The last 2 years we've gone on vacation for 2 weeks and put on about 9,000KM (5,600 miles). This allows for a few 'rest' days as needed. This year, since my wife has never been, we're planning on heading south to Vegas, west on the Lonliest Highway, and then north on the Pacific Coast highway.
Of course on the roads less traveled and without traveling the same road twice.
All that being said, we've also just picked a direction and taken off for 3 - 4 days with no plan except to ride.
Bottom line in my opinion is no matter what you decide to do, enjoy it!