Both. I use to do a ton of thumbnailing, but nowdays I do a lot of my layouts right on the page as I go along, I dont have the time and patience to thumbnail and then transfer that to a page- especially working digitally. I'd probably still do it if I were working traditionally, because I could scribble out a thumbnail, blow it up and lightbox it with blue line before throwing ink on top of it, but it seems a little extra when working digital.
While I have shortened my page creating process digitally, I've now started do more in the writing/scripting stage. I find that doing a little more scripting helps me to better set up my pages to do the art without leaving everything to interpretation; I still keep things loose, but the detail of setting it all up in the beginning allows me to finish it working loose.