Typically a simple greyscale conversion isn't enough. What happens is that darker things would sink into the skin and lighter things would pop up. This is compounded by things such as uneven lighting in the photo reference, etc. In general, if you have a good base texture to work with, you can make do with simply converting to greyscale and then adjusting a few things.

Moles and freckles shouldn't sink into the skin. A mole might even be raised some so they should be neutralized.

Eyebrows shouldn't sink into the skin (dark eyebrows would do so in a simple greyscale conversion) so you have to invert and do a lot of tweaking about the brows to make them work.

Nipples shouldn't sink so you have to do the same thing.

Same thing with body hair of any type.

Then there is the uneven lighting which has to be killed. What might look okay on a texture map can look like garbage on a bump map.

Hope that helps some