JIm,
Yes, wash flows better on gloss finish. Your plan of putting on a gloss coat and applying a dull coat later is good. Plus, you get the extra benefit of ease in applying the decals onto the glossy surface.
Is it necessary? Well, washes give realism to kits in that it creates subtle shadows on panel lines, for example. But hey, modeling IS for you, so the necessity for a wash or a matt coat afterwards depends entirely upon your liking. As has been said, the only person you have to please is yourself.
Hope this helps.