Braking resistors aren't required on a spindle motor unless there is a heavy inertia load on the motor. End mills or tool bits don't have the mass to do this. Motors act as generators when the rotor is being spun by an external source after the power is removed. All VFD's can absorb a fixed amount of this voltage. Braking resistors absorb motor's feed back voltage when it exceeds the drive's capability limit. Unless the drive deaccel is set for 0.1 seconds a braking resistor is a waste of money, wiring and panel space. A 5 or 10 second deaccel rate will not trip the drive.