On brookie streams, my most common rod is a 7'6" 4/5 weight fairly fast action graphite rod and I put 7 wt line on it. Many say overweighting slows a rod down, which is true if you're casting 30 ft of fly line. But when you're casting far shorter distances, IMO the heavier line weight merely loads the rod like it was supposed to be loaded. For me, in thicker stuff, being able to generate line speed with little or no backcast, and very short lengths of line off the tip, is really what you're looking for.
In more open surroundings, or for pickier fish where you'll be throwing smaller flies, then the advantages of faster action and heavier line weights lose their luster, and the soft action-low weight rods come into their own.