I can only speak from own experience that for holiday's and in our case some day's over in a high risk area for heart worm in the States, our vet didn't found it necessary for our dog to have the medication, only a test once back home to see if the dog wasn't infected which was never the case.
Living here is a totaly different case and although I am alway's very much cautious with injections and inoculations and certainly if you have border collies and aussies like we do who can respond very badly to certain drugs (genetically defined), I do give every one of my dogs here all necessary inoculations.