Usually the topcoats are recommended over the epoxy primers because topcoats have UV-inhibitors in them, while the epoxy does not. I don't think it's a requirement to cover ANY of the epoxy primers, however be prepared for chalking or discoloration if you don't.
BTW, when I took my refresher course for Dupont Chroma-Refinisher a year ago, the instructor couldn't speak highly enough about the new Dupont Epoxy primers. However, I never find the Environmentally-friendly paints as bullet-proof as the killers, and I've yet to use them (I have so-many gallons of POR15 that I'll probably never use it all!).