I paint the interior of the body shells to replicate the looks the 1:1 car would have.
Usually I paint them in primer color, with body color overs´ray where needed and paint the inner fenders black, when they are this way on the real car. On a '57 Ford, or '66 Galaxie, the inner fenders are separate parts from the body, and come painted a semi gloss tone of black from the factory. A '62 Thunderbird, is a unibody car, and will have the inner fenders body color on the inside of the engine bay, and in primer, body color and undercoating on the inside of the wheel well.
The exception is the headliner, that is painted to go with the interior color option I went with.
Example:
1957 Ford:
1957 Ford Fairlane 500 Club Victoria. by CCCP Digital Studio, on Flickr
1966 Galaxie:
1966 Ford Galaxie 500 7 Litre by CCCP Digital Studio, on Flickr
1962 Thunderbird:
1962 Thunderbird - Done!! by CCCP Digital Studio, on Flickr