Most of the wheel building resources I've come across mention destressing by pressing down on both sides of the rim, rotating a few degrees, and doing it again. I turn the wheel over and go around again. I use a block of wood on the bench to protect the axle and bearings etc. I've done it many times and it seems to allow spokes to unwind; I put the wheel back in the stand and continue truing. That's cool.
The instructors at the coop and the owner of the LBS both insisted that if I do not have the "6:00 O'clock position" of the rim supported on the floor, I was risking trashing my rim...?
This doesn't make sense to me. The spokes are already under a fair amount of tension on both sides and if the wheel was going to taco, it would do so whether or not the rim touched the floor at 6:00.
I have over tensioned wheels, but I've been able to release them, start over, and get them dialed in.
What do you think?

Not necessary....

Just give parallel spokes on both side a hard squeeze with both hands at the same time.

Do for two rotations in between tension cycles, and one last time to verify wheel is really done.

For 32 holes wheels for example, do a 16 count as you are squeezing 4 spokes at a time - 2 parallel each side. That'll be two rotations...

=8-)

In my opinion these two operations achieve different things.

Rubato's lateral rim flex allows overwound spokes to unwind by simply flexing the rim to the point where the tensile stress on the spoke is low enough that the nipple can turn freely.

mrrabbit's spoke stressing forces each spoke of the pair against its cross spokes hard enough to overcome some residual stiction at the cross and / or deform the spokes slightly so they cross more evenly.

I prefer to perform both sets of operations.

To the original question: No, you are unlikely to damage the rim the way you are doing it. Supporting the rim at 6 (and 12) oclock while stressing it at 3 and 9 actually increases the stress on the rim because it isolates the spokes better - if the rim is unsupported some of the force you apply is used to unstress the spokes near 6 and 12 oclock but since it won't be enough to release the nipple the energy is "wasted".

https://draco.nac.uci.edu/rbfaq/FAQ/8c.1.html

I'd ask them why they think that. Then I'd ask them if it was from personal experience or something somebody else told them. Regardless of what they say I'd store their answers as data points in my memory bank and do whatever I thought made the most sense.

I'm pretty set in my wheelbuilding practices but I still think that it's good to keep an open mind to other people's ideas.

I prefer to put the wheel on the bike and ride 50' and then remove it and check it. As soon as you put your body weight on the bike and ride, you'll hear the stretching and setting noises from the spokes.

You can replicate the "supported at 6 and 12, stressed at 3 and 9" on your lap by:

• 6 o'clock, supported by knees
• 12 o'clock supported by hands (grip the rim like it's a very narrow handlebar)
• pushing down with the elbows/forearm at 3 and 9 o'clock

Very satisfying noises ensue.

DoctorBike

I often use the methodology you described, centering my hands on the positions of the spokes I want to unload so they can unwind. I suppose a person could overdo it to the point that they sprung the rim into tacoform, but from simply observing the spokes, it's easy enough to see them reach the unloaded point, at which point one can release pressure and move to the next pair. If I want to particularly help the topside spokes bite into the hub flange on a brand-new hub, I can also do that routine but with my hands centered on the positions of topside spokes, so the force is focused on just two topside spokes at a time.

On new pricepoint bikes getting assembled, I just fill the tires to full pressure, prop the 6 o'clock up on my left thigh with the 12 o'clock on the edge of my workbench, and apply downforce at 3 and 9 o'clock. The first side gets this four times with a 90° rotation between them, then flip and repeat but starting from a 45° offset location. Quick & dirty, but it gets the plinking phase out of the way. Visual demo at about 6 minutes and 7 minutes into this YouTube clip: https://www.youtube.com/watch?v=UDaF2J35kbk Sorry it's in fast-forward, but you get the drift

As clearly stated in the link posted by davidad...

All the "other" stuff such as clock positioned manipulation (bending which is risky), light squeezes of parallel spokes, walking on the spokes and rim, etc....

...are either unnecessary, don't work or risky because of "control" issues.

As noted clearly, a machine pressing the hub in various directions (machines operation under controlled conditions)

or

hard squeezes of parallel spokes will suffice.

=8-)

...of course I hope you folks charge your customers what is necessary to cover potential damage liability and the time necessary to do stuff beyond what will suffice.

=8-)

+1

Between sqeezing the spokes and actually rolling on the wheels, I'd say that's enough to cover it IMO.

Mind you, when I first roll on a freshly-built wheelset, I tip the bike over each way as far as I can.