I would say no, you don't usually have to unless you want to have the photo look a certain way for your artistic style. The more foreground in your photo the more post processing you'll want to do I would guess. This photo is two stacked images taken back to back, with zero post processing of any kind (and automatic white balance in camera):

http://www.flickr.com/photos/ericols...2909/lightbox/

If you shoot at high ISO, then you may need to do some noise reduction. It's tricky with stars though, because to get rid of the noise, you often lose a lot of the faint stars that make the photo look so cool. I've heard that StarTrails is a decent program for stacking multiple exposures, but I have not used it yet.

I think if you get more into looking at nebulae and stuff then most post processing would be needed as well.