I never taste my wort. I don't think it is a good representation of the finished product.
My previous wiseguy remark aside...
I think one of the more difficult aspects of learning how to evaluate the flavor profiles of beers is learning how to recognize the characteristics contributed by yeast. Tasting the unfermented wort before, and the finished beer afterwards is very helpful in understanding that.
I like to taste my beers from start to finish, at all phases, to try and better understand how it all works. I am slowly gaining an understanding, through trial and error, how small and simple changes affect my beers throughout the process. True, it does taste much different at each of the steps, but understanding the changes will ultimately help me make a better beer.