I don't think I would shave her head... Maybe trim the wisps a little bit... but not shave. lol. I was lucky though, and my DD was born with a pony tail... heck, she's only 2 now, and her hair is nearly waist-length!
I have noticed that the hair that USED to fall forward, and was trimmed as bangs when she was younger, is quite a bit longer and thicker than the hair on the back of her head... I have debated with trimming the bottom half of her hair to get it to grow like the rest... but I never do because I love her baby-fine curls... even if they are hidden under a layer of thick, straight hair. :P
I know that my own hair used to be stick straight, but when I cut it to my shoulders in the 5th grade, it grew back in super curly... My little sister's hair was super thin, and she cut it boy-short and now it's crazy thick! Plus, the whole half&half deal with my DD's hair where ONLY the parts I have trimmed at some point are thick while the rest is still baby-fine.... I know people will say that cutting hair has no effect on how it grows, but my personal experiences tell me differently. :)