Cutting Inside of Top Lip for Rhinoplasty - Should I Be Nervous?

i don't think that you need to be particularly nervous.  this is not a frequent technique but it is a well described technique. 

the upper sulcus incision is an approach to the depresor septi nasi muscle.

if your surgeon states that he is going to do this i would imagine you have a plunging tip with smile, an oblique angle between your lip and columella (lower part of nose), and perhaps a "short" upper lip/gummy smile.  - you mentioned some of this in your question.

if these or even some of these are true, that approach and release of the DNS muscle may very well be indicated.

i agree with the other respondents that if you are nervous about it that you should get more clarification.  but dont the fact that they dont use this technique or have not heard of this technique shake your confidence in your surgeon. it is not experimental and it is not a relic either. 

it may turn out that your surgeon has a very sophisticated and science/anatomic based approach to your particular surgery

     The nasal depressor septi release (to prevent tip plunge during animation) can be performed from an intranasal incision or an intraoral incision.

From your description of the procedure, it is difficult to determine why your surgeon would recommend cutting inside your upper lip. Typically this is done inside the nose in order to drop the upper lip some or drop the tip. I would ask your surgeon his reason for this approach and what to expect when he/she does it. Good luck.