5 thousandths, or .12 mm - is enough to cause trouble. Depth of cut is .12mm for circuits.
See Shapeoko discussion - [1]
Latest Shapeoko uses GT2 9 mm
Most RepRaps use GT2 6 mm

Calculations

Positioning accuracy of GT2 belt - Say 1/2" pulley. Circumference about 40 mm. 200 steps per revolution = .2 mm positioning - but with 16 microstepping RAMPS standard - we get down to 0.01 mm positioning accuracy, or 0.5 thousandths
Chart at [2] shows positioning accuracy of GT2 belt to be 0.0003 - about half that of the RAMPS 16 microstepping standard
The question is - how much does the belt stretch upon motion?
Why is there no resource that states belt stretch as a function of force?
Contact Application Engineering Department - this bible - [3]- Tel: 516-328-3300

GT2 Belts