The side area does affect it and if you have a wildly odd looking model then you need to worry. But for more or less normal designs you can forget it. The side of the nose cancels out the side area of the tail generally.

What we are actually considering here is the vertical tail volume coefficient. That's the factor that uses the size of the fin and length of the tail to arrive at a number. So a short tail needs a bigger fin and a longer one a smaller fin. Dihedral and wing placement also comes into play. A polyhedral or old timer V dihedral with lots of anglel needs more fin area to avoid dutch rolling and a high wing model can use a couple of % more than a low wing model.

Also it's not totally specific down to a single % point. There's actually a fairly broad happy range for most designs. So call it anywhere from 5 to 10% for a more or less normal fuselage based on wing placement and dihedral. And then there's that bit that Dick Hanson alludes to. With enough power and at very low speeds you can get away with murder. 3D parkflyers are using some huge fin areas.

As for fuselages the parkflyer and ProBro designers are making the fuselage shapes such that the 25% side area point of the fuselage matches the 25% point of the wing so the model will more or less fly in knife edge with the same trim as it does when "flat".