For casual no specific timeline trips and pre smart phone we started our day fairly early and around 4PM we would stop and look for a motel/hotel. If they were booked up (or too many $$$) at that exit we went to the next, always had a room before dinner. There was some pre planning if we were travelling in really rural areas as motels/hotels are not a dime a dozen. We still do something similar now but with a smart phone we can look at rates nearby and decide from there, sometimes book online just before we get there. My logic is we don't know how far we will get that day or we may make unscheduled stops for interesting things we find along the way so the flexibility pays off.
In other cases we will book in advance if a midway point is also a specific destination.
In other cases we will book in advance if a midway point is also a specific destination.