Once the image has been chosen, getting the value study (lights and darks) is only a necessary evil. The important goal is to provide a dramatic setting for the subtle loveliness of transparent colours. My favourite sections of finished paintings are inevitably those areas where multiple layers shimmer, such as Jon's glowing shirt as he fished at sunset.
The guideline which has evolved: I take reference photos on bright sunny days; on cloudy days with diffused light, I stay home and paint.