The above are some quick studies I had done for a concept painting for a client awhile back. I did these from the reference as a kind of prep for the final painting. The image on the right's shadows are a bit dark then they'd probably actually be, but that's do due to the photo ref. Normally that wouldn't be an excuse, but I changed it a bit on the finish painting.
Basically, by doing studies of the references individually as is, I bring that knowledge into the final painting to enhance it significantly. I try not to spend too much time on them, just focusing on getting good value and color. I think by doing this it allows me use the reference more effectively and prevents internalizing/drawing out of my head and stylizing too quickly.