Should you overload that stiff rod of yours? The answer is actually pretty simple. Fly line is weighted in order to load or flex the rod. Therefore, the first 30 ft of 5 wt. line weighs more than the first 30 ft. of a 4wt. line. With that said, if we have 30 ft of a 5wt. line out on a 4 wt rod we will be making that rod flex more than if we had it lined up with a 4 wt line. With today's fast rods that can be stiff as a broomstick, this will help. There are definitely fast action rods out there that fail to flex with just 30 ft. of line on them. This is where overloading the line weight will be essential and highly recommended.

Overloading a rod can also give the caster more feel. Obviously, the added weight will load the rod more and be detected easier and let the caster know when to start the forward cast.

I've overloaded rods as much as 2 line weights. But this was only with what I consider "bad" fast action rods. If I can be critical of poorly constructed cheap rods, this is the area of contention, but that's another topic I can address some other time.

The only time I've experienced a rod to be overloaded to a fault was when I was carrying over 65 ft with the false cast and the rod was losing its "tracking" or accuracy. Basically the rod tip was telling me "you've got too much weight out there for my tip to guide". But besides this, I would recommend overloading your stiffer fly rod, at least just to see if it helps your casting.