Females:
Spaying before the 1st heat reduces the chance to zero of developing mammary cancer.
http://www.veterinarypartner.com/Con...&S=0&C=0&A=584

males:
http://www.veterinarypartner.com/Con...&S=0&C=0&A=574

My personal preference is to spay a female dog before their 1st heat.
(spay 6-7 months of age)

No opinion on males accept to make sure they do not in pregnant
other dogs or if a behavior problem neuter before all the
harmons kick in.