Vaccination for rattlesnake bite:

Reasons not to vaccinate:

Although rattlesnake bites make dogs very ill and can do serious damage, even untreated bites are seldom fatal and generally cause no permanent damage. 

Rattlesnake bites don’t happen very often.

Immunization of any kind can cause an allergic reaction. Most vaccine reactions are mild and easily treatable. Rarely, they can be severe or fatal.  Sometimes se see local swelling at the vaccination site, but have neither seen or heard of other problems.  

Reasons to do it anyway:

If your dog has a high risk of being bitten. Certain neighborhoods have a lot of rattlesnakes. If you live where there are lots of snakes or if you take your dog hiking in the mountains and don’t use a leash, then vaccination is probably a good idea.  

If your dog has already been treated with rattlesnake antivenin. Rattlesnake antivenin is made from the blood serum of hyper immunized horses. A single dose can make dogs so sensitive to horse serum that a subsequent dose of antivenin is rapidly fatal. There are a few exceptions, but generally speaking, dogs that have been treated once with antivenin should never be treated with it again. The risk from a second dose nearly always exceeds the risk from the rattlesnake bite. Immunizing your dog eliminates the risk from antivenin by eliminating the need for antivenin.

If your dog has been vaccinated and is bitten by a rattlesnake, antivenin is probably not necessary.  Antivenin treatment is likely to cost at least a thousand dollars, so vaccination might be a good investment.