It is extremely irresponsible to simply not vaccinate your dog. There are some very serious outbreaks of parvo in the UK at the moment and dogs can easily die from this.
I do not agree with over vaccinating, but you cannot simply just assume that your dog is immune. If you do not want to over vaccinate or vaccinate unnecessarily, then the responsible thing to do is to get a titre test done to see if your dog is immune and only vaccinate if they are not.
My own dogs haven't been vaccinated for years as they haven't needed to be. However, I still blood test each year to check, I'm not prepared to put my pets at risk either from unnecessary vaccinations or allowing them to contract diseases they could be vaccinated against.