curious to how often do you all get your Zipp spokes tension checked? if the wheels are still true do you bother checking the tension of the spokes before a new season?
If it’s true, I wouldn’t bother.
To check spoke tension, you need a spoke tension meter…which generally aren’t that cheap: http://www.wheelbuilder.com/dt-swiss-tensio-analog-tensiometer.html
You can use the Park Tool tension meter which is much more affordable: http://www.parktool.com/product/spoke-tension-meter-tm-1
Your wheel can still stay true if the spokes lose tension somewhat simultaneously. My rear Zipp 404 started flexing a fair bit but was true, and the spokes needed serious tightening.