I'll always look at reference material to see if I can get whatever I am working on to look more authentic... With regards to C3PO I did the inside of the hands with some black detailing because Anthony Daniels wore black gloves with metallic finger pieces over that, and I wanted to capture that look.
Sometimes I might do a little something that is not necessarily screen accurate, but I feel that it makes the model look more interesting. But I'll never do anything that really takes away from the original. Just enough to put my own stamp on it.
If that makes sense...
For example with K.I.T.T, even though the instructions say to paint the roof and rear windows smoke grey, that didn't seem right to me. In photos from the show the over head glass panels seem to have a black tint.
So I've bought some transparent black paint (that seems to not make sense!) and I will airbrush on a tint to those windows, because I think it will look way better and can still be seen through.
And, the model doesn't come with door windows, so I am going to cut some 0.005" clear styrene sheet to fit to those openings and see if that looks better.
Sorry, this post was way longer than I thought it would be...