I would never take a chance with heart worm... I use NuHeart that is just for heart worm... small pill once a month... we do not have issues with heart worm in NoCal but I do not take a chance. I use it year round. In the area that there is frost/frozen for the winter you can use it just in the spring/summer/fall months.
For fleas/ticks I use frontline but only when I see fleas... again in NoCal not too much of an issue.. I need to use it maybe about twice a year and only when need or if I am traveling with the pups.