Only a good idea for faily fresh paint that you are doing something else with. For example, a grand sport stripe down the center or something.
Otherwise, if you already have the car sanded down and taped off, go ahead and put some fresh color on there (base coat is very thin) and then clear it. Your results will be much better. It will be a new paint job, not a new layer of shinny clear over an older fading paint job. Plus, straight clear might give you some fish eye problems with out the proper base being set.