I think you need to reserach this very thoroughly and decide on what is best for you and your dogs, also taking into consideration your lifestyle and therefore exposure to risk.
Pat's post was excellent, thank you for that.
Brian, sorry to worry you but there are just as many concerns about over vaccinating cats!!
Keeping the immune system healthy is very important - for ourselves and our companions, that is our first line of defence against infection/disease. So a good diet, exercise, fresh air, freedom from stress etc are all very important too.
ETA we always get hearts checked at EVERY opportunity - and it is essential this is done before vaccinating, also that their temperature is taken to make sure they are not fighting an infection. If the dog is off colour or showing any signs of less than perfect health, delay getting the vaccine.