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.

IPR is basically the creation of space between teeth to allow for them to be moved into a new position. The timing of IPR is very important because Invisalgn utilizes software that simulates the movement of the teeth to fabricate each of the aligners. If a new aligner is inserted before the space that it needs has been created, then the aligner will be attempting to move teeth into a space that has not yet been created.There will be no harm done if the space is created prematurely, however it's still best to follow the instructions that have been created by their computer-generated model. I hope that you have found this information helpful,Dr. C

IPR can usually be done earlier than suggested and as the other doc said you just need to make sure you do not overreduce.  You also do not want to wait and do it later than expected since teeth are not able to move if there is collision of the teeth