I used to be of the opinion that it is unnatural to neuter animals (and still am), but I got snapped into reality one day when Ellie's vet strongly suggested that Ellie be spayed. She was spayed at age six, so the risk of mammary cancer was not reduced, but the risk of pyometra was. If we hadn't done it and pyometra would have showed up one day, Ellie's vet is the one that was going to have to go in there and take her pus-filled uterus out. This is all while a raging infection is going on possibly in a dog's teenage years when surgery may be best avoided because of other health problems. Oh and if the uterus gets spilled on the way out, you can have a REALLY big problem. It's kind of difficult to maneuver in such a small dog's abdomen... So in the end I decided it was best for my girl and will continue to spay.
In the case of males, don't unaltered males have a 50% chance of getting testicular tumors (can't remember exactly)?
In general, I wouldn't go for a surgery or procedure just because some rescue organization who euthanizes daily says I should because they are not vets. I would rather listen to experts in the field about what is best for the health and well being of my dog. And in fact the experts in this field recommend it (but believe it should not be mandatory):
http://www.theriogenology.org/displa...barticlenbr=59