I usually use my sketching phase as my thumbnails as well- so the idea is very much laying out not just the individual panels, but also the overall page. My sketches are looser and more expressive than the final art. They help me think about composition and as @BoomerZ said, the camera view.
The sketching phase is vital for problem solving. It's how I notice if something isn't reading clearly, or can be placed better. Because sketching is quick, it's no problem to erase, re-draw, re-orient, until I get something I like.
After my sketching phase, I look for references that match my sketches as close as I can, though sometimes this also leads to re-thinking panels depending upon the reference photos I find. Now is a brand new sketch phase, which is more about following references to get the building blocks of the pose and anatomy correct. For this, these sketches morph into my final drawing, as I erase and add detail.
There is no "proper" way. It's just about whatever work flow works for you. But this is how I do it, if that is any help!