Choosing the right tippet can involve as rigorous and methodical of a process as Pat describes or it can be as subjective and whimsical as the preference of the guy who likes red cars better than blue ones because with red cars you can go an extra 1,000 miles between oil changes....
Both paths more often than not lead to the same destination. My guess is that this is true mostly because once you get past the stiff/soft thing to the top tier of brand choices (the half dozen or so brands that dominate the market), the perceived differences are probably a bit more about religion than actual differences in quality. Rio, Umpqua, SA, Orvis, Frog Hair, etc., they're all good.
I started out fishing Nylorfi (1970's Cortland brand) and thought it was the best stuff ever for a long time. Somewhere in there was this stuff called Aeon which to this day, I don't think has been equaled for uselessness. Simply awful. Then, when it came out, I started fishing Orvis SS and knew in my heart of hearts that this was the stuff I would fish for the rest of my life. Until I bought a couple spools of Rio Powerflex and wondered how I ever thought SS was worth anything. Then we were in the UP a couple years ago and I needed to pick a shop owner's brain about the best sections of a particular stream. I don't ask for information unless I buy something. He handled Umpqua, so I bought a couple spools of it, even though I had enough Powerflex in my vest and gear bag to consider listing it in our wills. Near as I can tell, it is as good as Powerflex.
I thought I was done moving like a fickle vagabond from brand to brand. But then, last year, I happened upon this stuff: http://www.trouthunt.com/estore/details/23983 I really, really like it and am pretty sure that this is what I'll be fishing for the rest of my life. Or at least until the next stuff to strike my fancy comes along...
As Pat sort of said, pay attention to the composition of your butt and transitional sections. What you use for these can make real and easily apparent differences in how your leader performs. But when it comes to tippet selection for general use, I'm pretty much convinced you can throw all the brands that sell for $4.25 a spool or more into a paper bag, shake it up and pick one and you'll have the stuff you'll be fishing for the rest of your life...
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)