When we change knifes in the chopper we weight each of them. We always mount the same weight on the opposite side of the shaft. so its always rotating without a imbalance.
(ok, if all of them are new, ofcourse we dont do it)
Had all new knives in mine this year but shook so had it high speed balanced and was surprised just how badly it was out on one end. It's kind of like getting your tires static balanced or high speed balanced. Things can change as the flails move out into place.
Also may want to check the lengths of bolts that are holding the knives on as well as the steel spacers that are in between the knives and mount stubs. There are different sizes that can be installed on these combines.