im with Jean on this. i had in done before her first heat cycle (right around 5 months) due to the concerns regarding mammary cancer. my vet, who i trust completely, strongly advocates this for anyone not wishing to breed.
here is some additional info:
Mammary Cancer
Race Foster, DVM
Holly Nash, DVM, MS
Drs. Foster & Smith, Inc.
The risk of breast cancer is almost eliminated in dogs that are spayed before their first heat.
Spaying greatly reduces the chances of a female dog developing this condition. In those females spayed prior to their first heat cycle, breast cancer is very, very rare. The risk of malignant mammary tumors in dogs spayed prior to their first heat is 0.05%. It is 8% for dog spayed after one heat, and 26% in dogs spayed after their second heat.It is believed that the elimination or reduction of certain hormonal factors causes the lowering of incidence of the disease in dogs that have been spayed. These factors would probably be estrogen, progesterone, a similar hormone or possibly a combination of two or more of these.