The AC maps (with addenda) are pretty good at identifying where lodging is available. Here you can pick a destination at lunch and call ahead; if they're full, you may have to shorten or lengthen your ride an hour or two if you're going to motel it that night.
+1. The maps are pretty thorough when it comes to lodging. If you are looking at a city/town park, you really don't need to call ahead. With places like some U.S.F.S. campgrounds, you cannot call ahead as they are not staffed. Even full private places will often find a place to stick you, such as on a lawn, although you might not get such ammenities as a picnic table and a fire ring that you would get if had a regular site. Hotels/motels may be a different story as many of them can be on the small side. Project activity in the area can also affect room availability. On the northern tier our group found at least two motels that were booked solid with long-term guests. One place was full of archaeology students working at a nearby site. The other was booked with construction crews working on a pipeline project.
+1 on what Cycle Safe says about facility SNAFUS. I was riding the Great Parks South one year and planned to get dinner supplies at a small grocery store near my planned campground. I decided instead to go a little off rote to a larger town with the hope of finding a larger selection of groceries. Good thing I did because the store where I had originally planned to shop had burned down a few weeks before. The event was so recent that it had not made it into the addenda. The experience taught me that it can be benneficial to hit up an area with greater resources and carry food for a while rather than count on that the grocery store in the small town 15 miles down the road where I plan to camp still being in business and not closed for the evening by the time I arrive.
Later in the trip I got to a U.S.F.S. campground near Telluride to find it closed for renovations. Same thing happened a few years ago in Montana. I had checked the web page for the campground and there was no mention of it being closed for renovations. When I got home I emailed the Forest Service and suggested that they might want to note the temporary closure on the web site. I was surprised to get a quick response which read "Good idea." I was even more surpised that they quickly followed through withthe update.
It tends to all work out in the end if you remain flexible. Personally, I like to have a "Plan B" each day if it looks like I might need it.
BTW...If you will be taking the L&C route option through the Bitteroots to Dillon then north, consider staying at the Bike Camp in Twin Bridges. Great facility with plenty of room, and there are lots of services in town, including a good grocery store--assuming it hasn't burned down.