For people who live 5 hours from the nearest fly shop like I do that is not an option unfortunately. It really has become a problem with lines as well. I just saw an ad a while back where they brag about being a full line weight heavier than the industry standard. Then you have the problem of the industry standard only covers the first 30' of the line anyway.
And brick-and-mortar fly shops are not as common as they once were.
However, there are several other factors that testing like Common Cents can't address - line diameter, suppleness, lubricants, surface finish, hydrophobic additives, and taper, to name a few. The difference in diameter of a WF6F and a WF6S can be considerable, and the greater the diameter, the greater the air resistance. Suppleness can markedly contribute to the casting characteristics of a line. Lubricants, designed to leach from the line over time, vary from line maker to line maker. The smoothness of the surface finish - pebbly, ridged, sharkskin, etc. is also important. Some line makers incorporate hydrophobic compounds in their lines, this alters the depth of immersion of a floating line and the resistance to pickup. And, of course, line taper - is the front taper six feet or ten feet, is the belly fifteen feet or thirty feet.
With all of those variables, it would seem to me impossible to get an optimum line for a particular rod without trial and error. So, even if you determine by Common Cents testing that you require a 5wt, you are still going to kiss a lot of frogs before you get a handsome prince.