The only reason to not spay or neuter is you want to breed your dog. If you do not breed your dog, my advice is to spay / neuter your pet ASAP. Young pets are ideally spay/ neutered between 6-9 months of age.
Neutered pets are typically much better behaved, and there is a reduced incidence of breast cancer when female pets are spayed, and reduced prostate disease and cancer & testicular cancer in neutered males. The only down side to spay / neuter is a reduction in metabolic rate which makes them more prone to obesity, but this can be easily managed by reducing food intake.