It doesn't really change anything for most artists. The writer for Lockhart does this to some extent in his scripting. It takes me just as long to do the layout he wants as opposed to deciding it myself. The writer for Goblins of Razard likes to sit down with me in person and go over the script and I thumbnail it out in front of her as we go. She makes changes right then and there, and in that case it does save a lot of time.
However, I would not give any sort of discount for having paneling done, as I don't charge for paneling anyways. Most people either A. Charge an hourly rate and do every step themselves or B. Charge a stage rate (separate charges for pencils/inks/flats/colors/letters/etc.) or total page rate.
For basic commissions I charge $10/hour, and for comic pages I charge per stage of the page. I break it up, because several times I've worked in a team, where the person who inks is different from the colorist who is different from the letterer. Sometimes, the writer will also do particular stages of the work to save money and sometimes time. (Division of labor means faster completion times)
In rare cases I'll do package deals that include x number of pages, character sheets, cover image, formatting and consulting for print and distribution. But in the end it roughly divides up to about $10/hour still. 
If freelancing was my primary job, I would be charging almost $15-20/hour. But its my hobby, and I have a different career that brings home the bacon. So you have to consider those things too.
