Brass throttle shafts seem to wear about twice as fast as the journals in the pot metal throttle body. You might expect to replace a brass shaft twice before needing to to anything with the journals. To check for wear in the throttle body, disconnect shaft end fittings and remove the throttle plate. Slide the throttle shaft 1/2 inch off center to position unworn parts of the shaft within the bearing journals. Then wiggle the shaft up and down to see how much freeplay there may be in the journals. A tiny amount of wiggle about equal to the thickness of a sheet of paper (.003 inch) is acceptable and serviceable. When in doubt use a dial indicator to measure the motion. Clearance and freeplay in excess of 0.010" begs for refurbishment of the journals. Clearance in the range of 0.005"-0.010" is a judgment call, depending on how much you expect to drive the car after installing a new throttle shaft.