As I fish glass fly rods, I am a terrible person to ask. Since the question was posted, I will interject my two cents.
I fish DT lines most often on anything 5wt and under. Right now, I am the owner of three glass rods that get fished, one is a 6'6" 5wt that I fish with an SA Mastery Trout DT5F line and it is still quick, for glass. Another rod is a 7'6" 6wt South Bend Classic IV that I fish with an Orvis Clearwater WF6F and the other is a Phillipson Powr Kast T80 that I fish with an Airflo WF6F. These rods have vastly different actions from fairly quick on the little feller to SLOOOOOOOOOOOOOOW on the Phillipson and changing lines isn't something I will be doing unless it is to toss a sinking line on them for deep water work. In my opinion, if the rod is too fast, rather than slow it down by overlining (one line weight is often sufficient, but I have gone two line weights up), why not buy a rod that suits your stroke better? There are a few rods that I have cast, the TFO TICR comes to mind, that are NOT happy with even a GPX on it. I had to go to a 7wt line on the 5wt rod to get the feel that I look for in a rod. You can do anything you want to as it is your rod, but I am of the feeling that rod builders place line weights on rods for casting rather than fishing. If you can cast 90 feet you can catch more fish, right? Wrong. The TFO TICR series is underlined by one or two line weights if you use the stated line, IMO. The reason for the AFTMA standards is just that, it is a standard. It is the weight of a fly line in the first 30 feet in grains. In this same manner, IMO at least, a 5wt rod should load with 30' of 5wt line out of the tip top, right?