Does the timing of IPR with Invisalign have to be exact?

Your question about IPR with Invisalign being done earlier by a couple aligners is a good one.  IPRs are done to create a better fit of the teeth at the end of treatment.  This is necessary because in many cases a tooth size discrepancy, between the upper and lower teeth, can exist and the IPR helps to balance that discrepancy.  As long as the tracking of the aligners is good, the IPR can be done a little earlier or later, but the doctor has to make that decision.

Inter-proximal reduction otherwise known as IPR is  the removal of a small amount of tooth structure, much like gently reshaping a finger nail to remove a snag.  It does not hurt the tooth nor hurt your gums/smile.  IPR is often necessary to get the proper alignment, good function and best possible smile result when going through Invisalign therapy.  IPR is staged and is often done before the time it is needed or at time it is required according to the Invisalign ClinCheck model.  It should never be done later than requested as that could limit the tooth movement thus interfering with the final result.  IPR is integral to the completion of an Invisalign case when indicated.  I often ask for it to remove black triangles that might occur when the crowded lowers are straightened.