Wow thats some really good thinking. I'll have to call tomorrow and get a price on a set of 17" fronts and 18" rears. Im' normally a guy who likes matching size rims but I don't think you'd notice with 1" larger and the same thickness rim size.
I'm pretty big on running the same size all around too, but I've come to the conclusion that a Duster looks more proportional with an 18" rim in the back. It's the same set up that Dust'Ya had, only it had a smaller front tire, if you wanted a visual.