New Discs - Balance Wheel?

Personally I wouldn't bother and haven't done in the past. The relatively light weight of the disc along with it's position on the wheel makes any weight imbalance negligible. JMO

 

The requirement for the disc's to be a good fit and machined to a very high std of sizing compliance (same thickness throughout circumference)
They would be perfectly balanced too !!

 

Ideally yes, from a true engineering sense especially if in practice you go really fast (rotation rather than road speed).
Practically no, within normal speed limits and road use.

You don't state the vehicle or intended use/speed (rotationally) of the equipment so a bit of assumption is required to answer.

 

There is no need to re balance the wheels after changing either front or rear discs, the only thing I would say is the OEM bolts come pre thread locked, therefore you will either have to buy a new set for each disc, or thoroughly remove all traces of the old thread lock and use Loctite or a similar brand, don't be tempted to re use the original bolts as they come out and just just re torque.