I've never used a leader straightener, but I've also thrown out a few leaders that may have been saved if I did. I use really light leaders (7x usually, or 5x at the heaviest) and they tend to have much less memory through most of them than heavier leaders. I'm also primarily a streamer fisherman so the drag of the streamer straightens out any curls in the leader pretty easily.
Then again, I used to fish primarily with dry flies and I never used a leader straightener then either. I found that pulling it through my fingers really tightly would straighten out a lot of curls, or at least make them straight enough to fish with it.
On a side note, does anybody know if flourocarbon or mono leaders have more memory? What about furled?
My passion for catching fish is eclipsed only by the fish's passion not to be caught.