Remember, that when creating morph targets for your character, you are building a musical instrument for the animator to play.  For the best facial animation, you are going to want to be able to blend expressions together.  then if need be, add additional keyframe shapes to really add the quirks to your character animation.  Believe me your characters won't have much character if they look exactly the same each time they smile, and at a certain point, you will get very tired of sculpting each keyframe.

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Instead, keep things simple and precise. It is possible to generate any facial expression by blending from a set of  well crafted states (figA.).    A working example of this is a Character made by Protozoa, Dexter Finnery Klaus.  Although he only has 6 morph targets, he can make every phoneme and run 30 fps in real time on an O2

Although additive morphing looks amazing, it is good if you understand how it is actually working under the hood. 
  To make a morph target, your software compares the position of the all the points to a "home" position, creating a "vector" for each point.  
  Then when you animate, the computer moves the point in the direction of the vector for each morph target. 
  You can take advantage or this if you make a morph target that looks good at both extremes at '0' you have the home target, at 1 you have one target, and by moving in the opposite direction '-1' you get another usefull target. (see fig above).