I don't think a clear coat is necessary to protect your color coats.
If you do opt for the clear coat, be sure to follow label instructions carefully. Recoat times are for successive coats of the same paint; covering with clear may require waiting for the color coats to fully cure prior to application, which for a lot of rattle can paints is perhaps on the order of 10 days.
The one time I went for a clear coat I used the same material in clear, hoping to get a nice gloss coating that would give a 'depth' to the finish. In spite of waiting about what the label instructed (recoat within 15 minutes or after 10 days), the clear still fisheyed to an unacceptable extent. I don't know if I could have clear-coated within the recoat time, given that it was the same material. Perhaps an experiment to run sometime in the future.
So, whatever you do, observe the recoat instructions (and, of course, surface prep) - they are there for a reason.