[WendiS]

We neutered Riley at 3 and haven't seen any particular changes in personality or behavior except he stopped marking everything in sight. Which is a good thing because I only turned my head for a moment and he had his leg lifted and ready to pee on a bag of dog food at our local Grange Co-op, he did pee on the vets reception desk while I was writing a check and was becoming a serial 'marker'. I was really tired of being the pee-pee police. One of the reasons vets are so determined to have you neuter early is because so many people try to breed their dogs "just once" because he's "such a nice dog". We all know, or know about, enough backyard breeders to sort of understand the logic. Then there are the people that project their own masculinity issues onto their dogs. My breeder left the decision of whether or not to neuter (dog was sold on limited registration but no spay/neuter contract) but did ask that I wait until he was at least 18 mos old if we did. We have the same vet and she knew our vet would push for 6 mo's. Another reason vets like 6 mo's is because males dogs become more mature and can test owners and become a little ahead of themselves without an owner that can competently handle them.
Unless you've had behavior issues that need to be addressed (in which neutering can be one small tool, not the fix!) wait until your dogs is physically mature.
But of course that's just my 2 cents.
Wendi