Here's some references to check on spaying:
AVMA Collections: Spay/Neuter
Some points to consider from research:
Mammary gland tumors are the most common cancer in female dogs (3.4% incidence), with
> Female dogs spayed before first heat having less than 1% of getting mammary gland tumors later in life than other female dogs
> Female dogs spayed after 1 estrus but before the 2nd having 8% of the level of risk risk of dogs spayed later
> Female dogs spayed after 2 estrus cycles having 26% of the risk
Toy and mini poodles are among breeds pre-disposed to mammary cancers.
Spaying even as old as 9 years old still decreases the risk somewhat.
Testicular tumors are 2nd most common cancer in dogs, with 0.9% incidence.
> Castration is curative (obviously)
Prostatic tumors in dogs has an incidence of about .2 to .6%
> Castrated dogs are at an increased risk of prostatic tumors (2-4 times higher risk)
Poodles were one breed named as being at risk for cardiac tumors, with an overall incidence of 0.2%. Risk in increased in gonadectomized animals by about 2times.
Orthopedic abnormalities - gonadal hormones control the fusing of the long bones. Spaying or neutering before this fusion of the bones delays the closure and causes the bones to grow slightly longer than normal (not visible, but clinically significant).
Spayed females (of any age) do have a higher rate of incontinence (4.9-20% higher). One study found the risk to be higher when spaying occurred before 3 months of age. Otherwise, there is no research evidence to link time of spay to risk of incontinence.
Anyway - the information goes on and on, and in the end the 'best' time differs dramatically based on breed of dog and the different risks that the breed is predisposed to. After lengthy discussions with people I trust and my own review of whatever papers I could find, I've made the reluctant decision to spay after 1st heat but before the 2nd heat for my female standard poodle (reluctant because I didn't want to deal with even a single heat). This will still significantly reduce her risk of mammary cancer (her greatest risk), not as much as spaying before 1st heat but it's a trade off with the possible orthopedic problems due to the long bone fusion issues. Incontinence will be a risk regardless of when I have her spayed.
In any case, the risk of any disease is just a game of chance. Odds are just that - odds, not set in stone. And there is no overwhelming evidence of any significant potential damage in either early or late (or never) spaying or neutering. Which means we are all just left to weight the pros and cons and decide what makes the most sense...