Hi Gene,
That's an interesting thought, here's what I can share,
1) I haven't heard of anyone bothering to "pre-stretch strings".
I wonder how stretched out you can get them with your hands alone?
It seems to me to take a while for strings to stretch, days of regular tightening (they keep stretching, but it's really slow).
I'd also worry about ruining the intonation, since it might place uneven stress on them.
If I was going to pre-stretch, I'd use weights. But that would require researching the breaking strengths of each of the strings, to avoid using too much or too little weight. [I believe nylons are in the 5-16lb range, but it could be a little higher, not much though]
2)Why not simply put them on, and get them stretched in the usual gradual manner, and if you find there's excess string, simply take the strings off, trim that amount off, and put them back on? You might need to do it more than once, but it should help you zero in on the right length.
3) Are these wooden pegs, or metal friction tuners? I ask because if you're talking about wooden pegs, I have seen the opinion that they work better if the string windings reach all the way down and actually press firmly against the wood. But...
I've also seen the seemingly contradictory opinion that it's easier to tune them if you push them back out a little first (to loosen the peg), then when they're in tune, push them in firmly again.
This is seems to contradict the first tip: how can you push it out if the string is pressing against the wood?
Maybe they aren't contradictory, maybe you can press it out a little even with the string there.
I haven't had experience with wooden pegs yet, so I don't know.