The insulation won't impede the electric field in any significant way. It blocks the motion of charged carriers, but the electric and magnetic fields can exist without that mechanism.
Stripping the insulation will, as you suspect, result in oxidation of the surface of the copper. While this patina does repel water, preventing further corrosion, at RF the current in a wire tends to travel along the surface instead of the interior. If the surface has a higher resistance, this will impede your reception (and/or reduce your TX power).
The best bet is to leave the insulation on, although if exposed to the elements, you'll want to be sure your insulation is rated for that. Ultraviolet light and weathering can make most plastics brittle.