Yarg. Yet another wheelbuilder who doesn't know what stress relieving is, but confuses it with windup spokes.

Regarding what stress relieving is, read here, or better Jobst Brandt's book,"The Bicycle Wheel, 3. ed".:
http://draco.acs.uci.edu/rbfaq/FAQ/8c.1.html
http://yarchive.net/bike/stress_relieve.html

Pressing down on the wheel sideways can easily destroy the wheel so I wouldn't do it, even if it was a good technique to remove stress in the spokes. Use Jobst Brandt's method of squeezing two pairs of parallel spokes instead to remove spoke stress. And avoid spoke windup all together by backing off the spoke key a 1/4 turn every time you turn it a 1/2 turn.

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Regards