There are actually 2 answers to this:
If your machine is a 10,000 rpm machine then the answer is yes, you’ll need balanced tooling (grade G2.5).
So having said that, if I have 6000rpm or 7500rpm spindle then I don’t need balanced tooling!
This is true, but there are advantages to using balanced tooling on machines without a 10,000rpm spindle.
Balanced tooling allows you to achieve better quality parts with better surface finish, as the tool will be running truer.
The other advantage is that tool life will be improved as the milling cutter is running much truer and therefore cutting more equally down the flutes or across the tips.