Where You Live
Vaccination should be relative to where you live. This may determine if a dog shot is necessary or even effective. Sometimes vaccines will protect against one strain of a virus, and not another. For example, leptospirosis varies from area to area, and most vaccines against this disease are area specific. Likewise, a vaccination against Lyme disease in an area where it is not existent is useless.
You should always consider your environment and which diseases your dog may be exposed to. Additionally, you should check the local laws. It's required in some areas for all dogs to have a Lyme disease vaccination, and this is due simply to its prevalence.
The rabies vaccine is the ONLY federally required vaccine for dogs in the United States.
Combination Shots
Many experts advise strongly against allowing your dog to receive combination shots. These are rounds of vaccinations that inoculate your dog with four to eight vaccines at once. Many claim this is an economic and convenient way to handle all the vaccines your dog may need. However, some argue that this method has been linked with serious health problems such as many autoimmune diseases. Supporters of this theory believe that inoculating with multiple vaccinations at once will stress the immune system into breaking before the vaccines fully take effect.
Combination shots are still widely used, and the idea that they are dangerous is relatively unaccepted in the veterinary field. Some who subscribe to this belief, however, will administer each vaccine a couple of days apart, instead of applying them all at once.
Pre-Existing Immunity and Early Vaccinations
It is useless to vaccinate against a disease to which your dog is already immune. Puppy immune systems develop rapidly, so having a titer test done prior to vaccination is wise and may save you money. A titer test checks the number of antibodies to a particular disease or virus in your dog's bloodstream. If antibodies are present or of a high number, it indicates an immunity to the disease, and a vaccination likely isn't needed.
Some veterinarians push to have vaccinations occur before a puppy has naturally developed its own immunity to a disease. As vaccines are a large part of veterinarian income, you should check to make sure the vaccine is really needed, as some vets will unethically abuse this system.
The three diseases which puppies are vaccinated against at approximately nine weeks are canine distemper, canine adenovirus and canine parvovirus. Each of these will usually take three vaccinations a few weeks apart to ensure full protection.
Early vaccinations are sometimes also unwise, as the effectiveness is diminished. Most vaccinations given in a puppy under nine weeks are usually not effective. However, some newer vaccines, like GiardiaVax for giardia, can be given in puppies as young as eight weeks.
When choosing what vaccine to get and what to eliminate, research is key to knowing exactly what your dog is at risk for, and what is required by law.