If they are not going to be shown, health tested, and potentially bred, spay and neuter away.
Most responsible breeders spay bitches after their last litter (usually around age 6-7) because of the risk of pyometra, realizing they are missing a lot of the cancer prevention. In the very least, there will not be ovarian cancer.
Many older stud dogs are neutered because of prostate problems.
6 months, however, is not always the best age. There are studies in Rottweilers, where bone cancer is highly prevelant, that have found leaving the dog intact into adulthood and spaying/neutering later may be a good idea. For Maltese and most other breeds of dog, that is not an issue. Even where it is, the decision should not be made lightly or blanketed for all owners of the breed. Other cases where waiting may be indicated is female dogs with "tucked" vulvas. Letting them go through heat can help pop it out and decrease long-term problems and the need for surgery.
For the average pet person, 6 months is just right and better overall for everyone involved.