When to update noms?/

There will probably be many examples given before this thread dies out, but I feel like the short answer to your question is... when the origin changes.  At least that's when I click yes.

Say you programmed a part in it's entirety.  Then later you decide to insert an alignment in the center of the program.  And let's say the Z origin when you wrote the program was on the top face of a part and the alignment you insert in the center of the program changes that Z origin to the bottom of the part.  All points below that alignment, the Z location is relative to the top face.  So, if you're probing from top to bottom in a line you're counting down in -Z.  When your Z origin changes to the bottom of the part, if you do not click YES, then the points below that alignment are still gonna be -Z counting from the bottom now which will make the probe go below the actual part.  Clicking yes in this instance will change all points below the new alignment to the +Z equivalent so they probe in the correct place.

There will probably be many examples given before this thread dies out, but I feel like the short answer to your question is... when the origin changes.  At least that's when I click yes.

Say you programmed a part in it's entirety.  Then later you decide to insert an alignment in the center of the program.  And let's say the Z origin when you wrote the program was on the top face of a part and the alignment you insert in the center of the program changes that Z origin to the bottom of the part.  All points below that alignment, the Z location is relative to the top face.  So, if you're probing from top to bottom in a line you're counting down in -Z.  When your Z origin changes to the bottom of the part, if you do not click YES, then the points below that alignment are still gonna be -Z counting from the bottom now which will make the probe go below the actual part.  Clicking yes in this instance will change all points below the new alignment to the +Z equivalent so they probe in the correct place.

Thanks for the info. I think this answer is what I was looking for. Do you know if the point locations imbedded in the cad model are dependent on the current part alignment or if they are pre determined by the origin attached to the model when it was created? I'm also trying to figure out why having a slightly off location origin changes profile despite the fact it relies on cad for nominal info. Feel free to ask more questions if that is not clear. 

Thanks. 

For the first part of the question... if I understand correctly what you are asking, I suppose the answer could be either.  For example, the initial points placed onto a model immediately after importing the model will be relative to the model origin when the model was created.  After you create an alignment and move that origin, all subsequent points are relative to that alignment unless you recall the startup alignment.....

As for the second part of the question, I don't deal with profile so cannot answer intelligently.  But if profile is like everything else, A rock solid alignment makes for a rock solid program.  If there's error in your alignment, I just almost have to believe that error will be carried to your measurement results and maybe in certain instances even compounded.