If given a choice between having a good makeup artist on set or doing retouching in Gimp or Photoshop, which would you choose?  My own choice is for a good makeup artist.  Here's why;
     1.  Retouching takes time, lots of it if you have a subject with problem skin, or VWUW (vain with unwanted wrinkles). 
     2.  It is much easier to hide unwanted features under a good quality makeup than it is to clone or brush it out in post processing.
     3.  A good makeup artist will notice some things like out of place hairs, or an area that could use a bit of powder so time isn't wasted.
     4.  Having a makeup artist on the set makes the photographer look more professional.  It shows you care about your clients.