I do the same thing as Bob (as usual it seems...!). I've never modeled a real fret slot or real engraving in Rhino...no need to do it. RhinoCAM has an engraving function...no need to draw anything more than curves in this case (I do machine fret slots on a radius)
Here's the closest I've ever come to modeling a fret slot....i just did rectangular surfaces where the frets are and moved them above the fingerboard & inlay surfaces. I used an actual photo of a Madagascar Rosewood fingerboard that I'll be using as the texture to render the inlay design (which will actually be a 24 piece inlay)
Trev