I've never done a bargello but I don't see why someone couldn't use whatever width strips they darn well feel like! You would obviously want to make the strips work with the size quilt you're making to get the effect you want (I probably wouldn't do 6" width strips on a teeny baby quilt for example) but if that's the look you want then why not??
I'm already thinking about some of the lovely Cotton + Steel fabrics I've drooled over and wider strips...