I don't order them a little short, as I make them myself. I do make them a little short, but I don't pre-stretch them at all and I try to account for that when I lay up the string on the jig--that way when I leave them strung on the bow overnight to stretch out, I don't have to twist them a whole lot before I serve them.
I have ordered a couple strings ready-made, and used the standard 4" less than the nock to nock measurement on recurves and 3" less on longbows. They must have been pre-stretched a little, as they seemed to work fine.
I have never found anyone that could give me definitive answer on the "how much is too much" twist question, so I made my own rule! I try not to twist more than 20 times. So far so good, and I don't worry if it comes out at 25 or 15, as long as there is enough twist to hold them together, I doubt anything I shoot at can tell the difference, or cares--I know I can't!
RonP