X Y balanced drive motors.

[JAZZCNC]

So it would help to have a balanced spindle?

Personaly I wouldn't bother unless you know the weight is going to be quite heavy because it's mostly not an issue. The Mechanical advantage of a 5mm pitch screw combined with the bearings and motors phases creating a slight binding create enough resistance to stop it back driving.

Like Irving says adding a counter balance can be negative and If you have done your job correctly the motor should be strong enough to drive the Z axis in normal use so doesn't need any assistance. So if you do have heavy Z axis which back drives then a better way is a holding brake.

The only time you'd really need or want to use a counter balance is if the weight is so much the motor size required would be un-realistic or economical to do.
For most DIY jobs and esp router based machines it's not needed. Often you'll see it done on Mill's etc but usually it's done because they don't need high accel on there mills and prepared to accept the trade off.!!. . . .OR. . . There just tight arse's and won't stump up for the correct motors to do the job..