I am transitioning now but I got touch-ups at a dominican salon for years. I used to go to the same person, so she knew what I wanted (do the hair line last etc).
If you have a salon in mind. I would suggest going there for a wash and set and watch them do relaxers. I always did this. From what I noticed some places do a lot of over processing (apply to more than new growth or pull to the ends of the hair).
Once the lady I frequent was out and I let someone else give me a touch-up, She told me to tell her when it burned. I was like...No wash it out! Hello if it burns it's on too damn long IMO!
Some do a good job but I would suggest you go and watch and tell them what you want BEFORE they start to apply it.