Probably hard to say out of season, but I always figure if the tail board is clean your chopper needs balanced.  If chaff will pile up on it, it is okay.  I have one now that is in deperate need of a balance job.  You probably know that on the 9XXX combines the rotor could get loose on the shaft on the left end.  You can take a 2x4 and pry up on it to check for play.  Since you just had it balanced I would think that this has already been checked.  I have had one fixed before and later found that they just put 3 set screws in on the left end of the rotor and centered the shaft, then welded it all up.  Didn't last anytime!  I have had the best luck with Precision Balance.  If the rotor is loose, he will rebuild yours or exchange it.