If you tension a spoke to its yield stress, imagine what happens to parts of the spoke that have residual tensile stress from cold forming in manufacture and bending into place in a wheel. These locations must have already yielded by then and when relaxed must have the same stress as the remainder of the wire, typically that of the straight sections.
This can be observed in a tensile tester when the spoke is taken to yield stress. Regardless of prior shape, when relaxed the spoke is perfectly straight, its stress memory having been erased because it has plastically deformed (stretched) ever so slightly. I said TO the yield stress, meaning any part that previously had tensile stress exceeded that stress and stretched.
Let's not try to hide behind semantics on this. If you watch spoke wire coming of the spool for manufacturing spokes, it must be stress relieved so that it can be worked. To do that, the wire must be straight and on the reel with its residual stress it coils up when unspooled. To straighten the wire, it is run through the equivalent of degaussing by being run through a series of zigzag rollers with ever decreasing amplitude, both in X and Y. Stretching spoke wire to yield for manufacture would be ungainly while this method of rolling spoke wire as it comes off the reel accomplishes the same thing.
That depends on the material. Spoke wire is about as work hardened as it will get and does not want any heat treatment, especially when this can be done cold. I have observed the method at DT spokes and tensile tested their spokes to assure myself of the value of the process.
I don't know what you mean by "documented process" but there are other ways that do not change material properties and they are as mentioned.
That's why I prefer not to use Carl's pejorative of "squeezing spokes" but rather stretching them to a stress level that will cause yield in the highest stress locations. Squeezing spokes suggests a picture of someone hugging or pinching spokes. That there are high stress locations, ones near or at yield, is certain because spoke elbows take their final form upon tensioning. That forming is plastic and causes yield. Any tensioning thereafter guarantees that the spoke remains at yield at a portion of the bend. Not stress relieving was the main cause of early failures before stress relieving was introduced to wheel building. there were wheel builders who stress relieved without realizing what that part of their process was doing.
Stretching spokes causes highest stress locations to yield and be relieved. However, manually stretching spokes cannot remove all residual stress because stress cannot be manually raised enough.
Jobst Brandt