Don't think they are required for airworthiness, because lots of airplanes do not have any filler plates....even a few RVs I've seen. Seems to be mostly "for looks" kind of thing. And, maybe (?) some aerodynamic benefit.....although I'd really want to see test data on that.
They certainly be added later. Just make sure they are attached per plans, otherwise the centrifugal force involved can make them "depart the airplane" in flight.