I wouldn't wait for her first heat. With each heat cycle the risk of mammary tumors increases. If spayed before first heat the chance is almost nil.
http://www.merckvetmanual.com/mvm/re...ry_tumors.html
I would say if she is over 5 pounds, she is ok to be spayed...but you should ask your vet for his thoughts on this. This coupled with the fact you have had her since April puts her at probably at least 5 or 6 months...a reasonable time for spaying.
Then again, if all of her adult teeth are not in, I would hold off. Vet opinion is where I would go with this.
They are at a higher risk of periodontal disease if those deciduous teeth are not removed. Many vets remove them at a low cost when they alter the pet because they really are easy to remove and don't take up anesthesia time.