I did, even though I’ll have the headliner over the interior. But it seemed to go well in a session where I could coat the inside and the outside in one quart.
If you’re in a quandary about it, wait until you see if you have leftover varnish, then use it up inside.
I varnished my whole interior but I don’t recommend doing any varnish where you will be using contact cement that is not water base for attaching insulation. I did so with a solvent based contact cement and it lifted and bubbled varnish up wherever it was applied. The insulation held fine but whate a waste of expensive varnish.