After several hours browsing the forums and rebuilding a dozen times with qDebugs, it's time to ask.
All my <double> values appear with two decimal points only. This is a plain XP system, latest SDK, US English. No currency is involved.

A clear example is comparing a new <double> to the value in a QDoubleSpinBox with 3 decimals.
Their QString representations are
"100.001" and "100.002"
Their doubles are
100.0 and 100.0
The outcomes are consistent using any Qt conversion method and Qt display method, including direct output from qDebug(), QString.toDouble(), QLocale, QVariant, etc. (I said, I've been searching!)
Other test values? "123.456" is converted to 123.46 -- looks like rounding.
This seems like a simple environment setting that I've missed. Where should I look?
Thanks, Scott