I personally believe in spaying prior to first heat cycle. Here are some websites with reasons why:

ESPN- Fact Vs. Myth

Dog Breed Info

And this, from the Oregon Veternary Medical Association:

Should the female have a heat period or a litter before being spayed?
If your pet is going to be a companion animal rather than a breeding animal, then there are no benefits to allowing her to have a litter or to go through a heat period.

It is actually healthier for your dog or cat never to experience a heat as it lessen’s the animal’s chance of getting mammary cancer and decreases the animal’s stress and risks due to pregnancy and delivery.

Research indicates that dogs spayed prior to their first heat have less than a half of one percent chance of experiencing mammary cancer as compared to an eight percent chance after the second heat.

Cats spayed after their first heat have a seven times greater chance of suffering from mammary cancer than cats spayed prior to their first heat.