Is there even such a thing as a perfect skin tone?
People with different ethnicity have different skin colour plus lighting and background adds a layer of complexity.
As a photographer, what should we be trying to achieve in regards to skin tone?
I believe the answer here is "reality, seasoned with prevailing cultural biases".
It would be really nice if we could just do a perfectly neutral rendition: meter off of a grey card, using a white balance appropriate to the lighting and a camera profile that takes into account both the colour spectrum of the lighting and the camera's response curves. But that's rarely what people (including the photographer) want.
Obviously, you do want things like ethnicity to show through, but unless you are doing a study in physical anthropology, the bare, unvarnished truth is rarely what you're looking for. For instance (and without adding my own values to the mix), in many parts of the world you'd generally want people in your photos to have slightly lighter skin tones than they do in real life. Often there's a significant bias that's almost invisible to an outsider, but within the community it can mean a lot. And sometimes the bias goes in the other direction, either because there's a "tanned is healthy" attitude or as a point of ethnic pride (which is often a reaction to the "whitening" of other ethnicities).
So the real answer is "know what is expected locally". You don't want to inadvertently insult anybody by going against the grain. If all of the pictures you can see of the darker-skinned people you see around you appear to be lighter than "reality", then making those people look darker is making a statement contrary to local expectations. (The reverse also applies.) If all of the olive skinned people are pinkish in their picturees, or the pinks seem a little on the brown side, there's probably a reason for it that's deeply tied to local culture. Before you make such a statement, make sure you understand what you want to say and how other people are likely to hear it.
Much less controversial is what might be termed "corrective action". If you put an outdoor worker (say a farmer) into a suit (say for a wedding) at the end of the summer season, you're going to wind up with a picture that looks like a farmer in a suit. A healthy tan is one thing; a glow-in-the-dark red is quite another. Your job, in a case like that, is to make the glow-in-the-dark red into a healthy tan. Similarly, a blond Finn shot at twilight in February under natural light is going to look sort of Smurfish without a bit of warming -- great for the proverbial ice princess (and the right person can look spectacular with a pale complexion and the right lighting), but certainly not the right choice for everybody.