Balancing is really important in my opinion too. And the balance changes because the insert moves I think. So whenever I have some time (not often
) between heats or finals I re-balance the tires. It gives you better performance and out of balance tires make crazy vibration to whole car.
Out of 4 Proline Crimefighter LPR premounts I had one VERY BAD and one was a bit better. The bad one needed about 15-20g
I would never use metal to balance the wheels. Think about what happens if it flys off
I use ordinary modeling clay ie PlayDoh 
and secure it with a little glue.
I have something like this:
Put the wheel hex on it and balance the wheel between your fingers. Easy and takes no room from the toolbox.