I have built a dozen planes with Monokote and now I want to try some fabric work. I have seen some posts here that say you need to cover both open and fully sheeted areas. Riddle me this... Why? Why cover something like the tail area that will be fully sheeted and seamless? Why not just dope it and then paint it? Any thoughts on this appreciated.

Here's the riddle. The wood surface needs to be "stabilized". Yes you can fill the grain with dopes, primers and paints, but the wood can still expand and contract with temp and humidity changes. Balsa wood is very soft and can get dings and dents very easily even when the grain has been filled. Covering the balsa before painting will actually seal it and give it a protective barrier against the dings plus it will usually be a lighter weight finish to boot. It takes quite a bit of filler just to fill the grain on balsa not to mention the amount it takes to get a smooth finish. I prefer to use lite weight fiberglass cloth(.5 oz.)/ epoxy resin to seal solid balsa parts before finishing and painting.
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Yup. Epoxy thinly applied with lightweight fiberglass works well. You can also use dope and silkspan tissue, or dope and fiberglass. Or water based polyurethane and glass or tissue. The goal is to fill all those little hills and valleys that are present even in a piece of well-sanded wood. (ascii art follows....)
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Forgot - you can also use Koverall over the entire project, sheeted and open areas, adhered with dope, or water-based polyurethane. That works well too. I've used all these methods with success. There is no one 'best' way.

The end result of any finishing method will only be as good as the sanding job underneath.