Never ever ever ever stop if you can possibly avoid it. There are only a few valid exceptions that I can think of:
What is a roadtrip without music? Music is grand! It can serve as a way of timing your trip (at 70 mph, a 90 minute tape is aproximately 100 miles). It can serve as a way of amusing yourself . But music can be a force for roadgood or roadevil. In order to make sure you are in the rock and roll heaven group, always try to listen to long songs. After all, if you listen to short songs, 8 songs could go by and you'd only be 20 miles down the road. Get into a good Dark Star from 1972, and by the time it's over, you're a lot closer to the goal. In fact, I listen to the Grateful Dead so frequently when I'm on the road that the previous comment is usually stated "One set equals 100 miles"
Here's a fun game for the obsessive math types out there. First fix a point as being the official starting point of your trip. I usually choose the end of the entrance ramp of the first Interstate highway or main route I take. Once there write down the mileage on your odometer and start a stopwatch. For the rest of your trip, keep track of what kind of time you are making. At each fill up, compute the miles per gallon you are getting. Constantly try to decide how fast you can go without wrecking your mileage. :)
Look, I've just said that I listen only to the Grateful Dead (and Phish), never make any stops, and obsess over the kind of time I am making. Would you want to travel with me?