Greetings, Robot-
I don't believe there is a way to set the number of decimal places displayed by the crosshair tool, but (and this is only useful in a small set of circumstances) if you change an axis to Log scale (by double clicking the axis, then changing "Linear" to "Log") the crosshairs will display an extra decimal place for that axis.
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I'll be happy to submit a feature request for this-- if you use the crosshairs often I imagine this isn't a rare frustration!
Julian