While I'm glad to read the opinions of both Matthew and crazylady, and I respect those opinions, I would caution them about making assumptions. Both of their posts include phrases that make it appear that they think I am a still-wet-behind-the-ears SM ("as you continue down this path", "you will get over that (nervousness)"). In fact, I am a twenty-year Equity member and was involved in theatre for another doezen years prior to that. I agree that SMs should take classes and get experience in many areas of theatre and I have: acting, directing, producing, music directing, props, etc. I'm not trying to brag - many of our resumes are similar, I am sure, but I want to be clear about my background.
I have no problem differentiating between maintenance and acting notes. For me it's the difference between the job of a director and an SM. And certainly a production is going to grow during the run - performances and nuances change, audience reaction and size can affect the pace. It is the job of the SM to guide the production through those changes while staying true to the director's vision. However, I would not consider telling an actor to hold for laughs an acting note.
Part of the equation is how the director structures the process. I've worked with directors who wanted me involved in every artistic decision, from helping to cast a show to asking me to help an actor find the emotional truth in a scene. And I've had directors who only wanted me to handle to audition paperwork and tell them what blocking was previously done in a particular scene. Obviously, the manner in which I handle my job varies greatly in these two scenarios.
The competence of the SM can also vary. As an actor, I have worked with SMs who wouldn't know art if they were to go home and find their dogs playing poker (okay, bad example). Some SMs want nothing to do with the artistic end of things, prefering to stick with the organizational and technical side.
Perhaps it's the words I'm choosing that are causing the disagreement. While I would not give what I consider acting notes, I could have an artistic discussion with an actor. It's the difference between leaving a note on his dressing table saying something to the effect of "You are channeling the wrong emotion in scene 2," and asking the actor how he feels scene 2 is going, opening a dialogue about his interpretations and my view from the booth. And of course, as we all know, different methods work better or worse with different individuals. Our job is about finding the best way to communicate in each instance.