I leave the rifle assembled and using a crow's foot on a 1/2" drive torque wrench, I grab the wrench flats on the muzzle to torque and break torque. I trap the rifle between my thighs/ knees muzzle up. This presents the wrench flats at about chest/ neck level in order to comfortably wrench on. I use 40 ftlbs of torque. I've tested hand tight, 20, 30, and 40 and haven't seen a poi shift as long as some amount of torque is used.
I see more zero shift from bore condition and shooting position. To put it simply I haven't had to rezero my turrets in over a year on my 6 Dasher. The Marine Corp did a study about 15 or so years ago on "cold bore" effects. What they found is that the vast majority of what was being attributed to cold bore shifts in the group of shooters was actually due to "cold shooter". Guy's taking a couple of rounds to "warm up". I personally have had a rifle that had a cold bore shift that was very repeatable. But I found out with my gunsmith that the action was not square. It was a slab-sided action. The inlet in the chassis was made for the perfect action and the action was not. We bedded it and that cold bore shift went away. I think if a rifle has a cold bore shift it's a mechanical deficiency. The cold shooter thing is why we now use dot drill exercises. Basically getting after the shooters repeatable precision over the rifles accuracy. Position as a fundamental and the natural point of aim component of position building has the greatest effect on your zero. I guarantee torquing (no matter how light) a properly machined barrel on an action face with an integral lug will have less variance than our ability to repeat our zeros. The difficult part of testing this is separating your marksmanship affect from the rifles mechanical affect since you have no way of testing without you in the equation. I mean hell, we've seen a difference in chrono data between shooting from a bench, seated vs in the prone.
All this to make the point that the most accurate thing I can say is that I haven't had to rezero in over a year of matches with my barrel off and back on in between each match.
Where it gets difficult is three barrels for one rifle x 4 switch barrel rifles. I honestly cannot remember each individual barrel's zero offset to within .1 mrad for all those barrels across 4 rifles. I only maintain a 0/0 zero on one barrel per rifle. All the others are an offset. So far the best method is to write myself an email, subject titled the rifle name with the zeros for each barrel in the body. I always have my phone on me and Gmail's search function is pretty good.