...why pre-stretch a string? Seems like an extra step to me.
I do it as a courtesy to the customer. When someone sends me a measurement, it's for a shot-in string, and that's what I try to match. Plus it saves the customer a lot of time shooting a string in, when they could be shooting for tuning, accuracy, etc.
I can pre-stretch a string in minutes under around 300# of tension. I can also accurately measure the string by AMO specs (under 100# of tension) by just hooking my digital scale to the jig.
My jig also comes in very handy for serving the string, tying on silencers, etc.
Chad