Wow, this is so interesting!
I for one do lots of podcovers and, yes, I totally noticed that when I work on them after listening to the podfic in question it feels like a more complete experience to me, while if the podficcer says something like "Hey, I'm going to podfic this thing, will someone do me a cover?" of course I'm super excited but if I didn't have a chance to hear their work first and I have to base my cover only on the fic it feels a bit like a more detached project to me and I worry that the resulting cover may be less than perfectly fitting. Not to mention the fact that if there's going to be a podficced version of that fic it's a bit "unfair" to have to physically read the fic when shortly after I could have instead enjoyed that same fic in podfic form. :D - LOL, I'm so spoiled! :D
Though of course I couldn't well ask the podficcer to wait a week for me to do the cover after they finished the podfic. O_o Not to mention the fact that I'd be too excited and impatient to get on with the work to wait anyway.
So, yeah, something to think about...