I'm with Vulcan - depends what you're trying to do. I prefer a more muted skin tone on my orks, but I still want some variation, so I do: [black primer] > basecoat (
GW orkhide shade) > midtone (VGC sick green) > wash (
GW thraka green) > highlight (VGC goblin green) > wash (thraka, again). The first wash does a decent bit of shading, the final wash deepens the recesses and helps blend the layers together. The result is very natural (well, as natural as green skin can be...), in my oh so humble opinion.

Same deal for their leathers - basecoat > highlight > wash, as I want a more subtle blend.
If I'm doing something like
SM power armor, I'm going for starker line highlights, so I won't do a final wash over everything, since I don't
want it to blend together subtly.