There are already SO many threads on this topic, and most of them turn into heated debates. There are many pros and cons of spaying/neutering, and there is no real "best time" to spay or neuter. It depends a lot on what the owner thinks, what the owner is able to handle, the health of the dog, the breed of the dog, etc... Everyone has their own preferences and ideas.
I suggest you do a quick search through the forums and read past posts on this topic. Here are a few recent ones:
As Equinox said there are pro's and con's to both sides of spaying before or after the first heat. Do a google search for articles and a search on here for the loads of threads on the topic. Soak up all the research and information that you can, disscus it with your vet, and make and informed decsion that you feel comfortable with.
Personaly I would wait until after her first heat to spay. But this also means that you have to be very diligent whe she does go into heat. Whichever descission you choose to make it has to be your own and not what everyone else told or pressured you into.