Re: Tuning a 12 string?
When I put on new strings I always start with the guitar tuned and replace one string at a time, tuning after putting on each string to keep it in tune.
When I am done the guitar is in tune.
This method keeps the neck under the same stress and stable. I do it this way on all my guitars. Other people have their preferences. It shocks me when a guitar tech cuts off all the strings at once and the neck destresses instantly. The necks don't always come back exactly like they were after restringing. Buzzing etc.
I've heard this before & it's a misnomer and potentially scaremongering, the neck cannot destress instantly to the degree you describe, perhaps if you leave it stringless for a few weeks the neck may move a little but if it's truss rod is installed correctly (which even on budget guitars they invariably are) it can be brought back with a 1/8th a turn of the truss rod.
Guitars are sturdier constructs than most give credit for, and if anything the stress they are consistently under from the strings is more detrimental to the guitar than leaving it unstrung, as the countless owners of the ubiquitous broken gibson headstock will attest.
I don't think there's any rightway or wrongway but here's what I do:
Slacken all of the strings so when cut they won't take my eye out.
Cut them all at the bridge end & unwrap them from the tuners.
Dip a cloth in water with a spot of mild detergent (washing up liquid) & squeeze it out so it's barely damp & wipe down the guitar
take a CLEAN toothbrush (ie not the one you use to brush your teeth but a new cheapo on) an use it where the frets meet the fretboard & on the metal hardware, again dipped in the detergent soulution if neccesary.
if an ebony or rosewood fingerboard evaluate how dried out it is, if it is dry apply a couple of drops of lemon oil to it & rub it down into the fingerboard.
leave it as is for 2-3 hours & wipe off any excess lemon oil with a piece of kitchen towel.
string up & tune up
stretch the strings, ie pull the strings up and away 90 degress away from the fretboard/guitar top, you'll pull them about 1/2" before they snap back, do this 10-12 times for each string & retune. you may have to repeat the last step every time you play for a few days until the strings settle & hold tuning better
It works for me & it's an eyeopener how grimy and/or dry fretboards can get if left
Hope it's helpful to others