Originally Posted by
Rubato
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.
Originally Posted by
mrrabbit
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.
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".