Do you ever eat all the frosting first? Probably not...because then all you're left with is cake, and cake is pretty dry without that sweet frosting...
Which is why I save the face till last in a portrait! Let me explain...
My method for portraits is to finish the background, foreground, clothing, hands, feet and hair first, before working on the face, and here are all the reasons why:
If I start with the face, it will get dirty or smudged as I work on the surrounding areas, and that's pretty hard, if not impossible, to fix.
If I start with the face, then work on the background, in the end the values of my face will be too light (because I was working against a pure white background to begin with.)
If I start with the face, I'll be working on the hardest part, without any previous commitment. In other words, if I don't do a good job, I haven't lost much. Maybe that sounds like a good thing? I don't think so! If I have 20 or 30 or 60 hours into the piece by the time I get to the face, I'll be determined to do a fantastic job on that face, no matter what. No failure allowed!
But here's the most important reason why I won't work on the face till everything else is done: If I work on the face first, I've eaten all the frosting!!!
I'm a portrait artist because I love drawing faces. I imagine it's the same for you! If I draw the face first, all the really fun parts are done. All I'm left with is stuff I'm not all that crazy about drawing. All I'm left with is dry cake!
A few years ago, I drew a portrait, but didn't follow my own rule. I wanted to use the face for a lesson for the magazine, so I went ahead and finished the face. Two years later, the piece was still hanging around my studio unfinished...until I accidentally spilled coffee on it. I just couldn't get excited about an arm and some grass after I'd finished my favorite part.
So there you go...I hope you'll consider finishing everthing else before indulging in that sweet frosting - the face!