In my experience, anything art related (thumbnails and page layouts included) is the artists job. If I'm working with a writer, I personally prefer to have some level of creative control and hate getting over worked scripts. A lot of people want to describe EVERY little thing in every panel and that makes me feel like they're treating me like an art-making-robot rather than an artist.
In my best writer-artist relationship, our workflow went like this:
1. He would send over the script with page breakdowns, but no panel breaks. So basically just dialog and key action points. I'd give him notes at this stage if any of the dialog felt forced to me.
2. I'd thumbnail it out to see if his page breakdowns worked. Sometimes he would want to put too much action or dialog onto one page (which is a common problem for writers) and I'd notice the pacing wasn't flowing, so I'd ask if I could break things into two pages, which he was usually cool with.
3. Once he approved my thumbnails, I'd go to sketches and show those to him. If there was anything off about the characters or settings, like someone's outfit wasn't what he imagined or he wanted something specific in the background, he could tell me at this point so I could fix it.
4. Then I'd pretty much finalize the inks and colors. Luckily we never needed revisions past this stage since we took care of that during the sketch phase.
It's a lot of back and forth, but that was the best way for us both to be proud of the finished project. The work load is obviously skewed, since drawing is more time consuming and intensive than script writing, so I was paid a page rate for my work. I wouldn't feel comfortable illustrating someone else's story for free under any circumstances, since it takes so much time and you are providing a service to them... but that's another topic for another day.
Basically, art stuff is traditionally the artists job, but you should feel comfortable having a dialog with your writer. You can both work together to divide the pages and panels. You're a team!