Most vets recommend spaying by about 6 months of age - which (in most cases) will mean that the bitch never has a heat cycle. Since the risk of malignant mammary cancer is increased with every heat cycle, having the spay done prior to a cycle is a good idea. If you don't beat the first heat though, then I'd say any time before about a year old is fine (in time to beat her second heat). One thing not to do though, is spay a bitch that is already in heat - if she comes in before the spay, then you need to wait until its finished.