I like the advise about tippet rings. Since I bought some I can't imagine going without them again! I used to keep several spare leaders in my bag in a leader wallet, because eventually I'd cut so far. Back into the leader that it was just easier tone place it than to add 2x,then 3x, then 4x to rebuild the part of the tapered that I had cut back into.
Now I pretty much only buy 9' 3x tapered leaders (for trout at least). I will generall either use. Them or cut the tippet back to just before the taper starts, and add a tippet ring there. Then if I want to fish a streamer, I'll add a short section of 3x ,or if I'm fishing nymphs or dry flies I can add a surprisingly long piece of 4x or 5x and the leader will still turn over. I'll use the same leader all season or until it starts getting beat up.
Usually my streamer leader is short (about 8ft if I'm using my tippet ring system, but as short as 7.5 ft if I'm just using a plain tapered leader), and my dry fly/nymph leaders are longer (at least 9 feet, sometimes as long as 12 ft). 3x for most trout streamers, and 4x-6x for dries and nymphing, tending toward lighter for smaller dries.