Do you know your best hem length? Stand barefoot in front of a full length mirror with heels touching and toes turned out (1st position in ballet). Unless you ARE a ballerina, you probably won't be able to turn your feet out quite like THAT!
Now, do you see the diamonds that are created in this position? Those are the most flattering hem lengths for you. And you should have at least one diamond.
Wearing your skirt or dress so that the hem is at your diamond is the most flattering, however sometimes you can fix the proportion by choosing a shoe height that is more appropriate to your hem length.
A too-long dress or skirt can give you a dowdy appearance. If you have an outfit that you generally love, yet something is not quite right, it may be that the hem length is all wrong.
Wearing a dress or skirt that is too short for you can look juvenile, or something else, especially if worn with a higher heel. However, if you do wear a shorter length for casual occassions, wear a lower heel. That may be just the fix you need for a better proportion.

Any questions, class? 'Just kidding. I'm no fashion expert. I just play one on this blog.
(photos from photobucket)