My heart worm Rx is 9 months, from April to Dec. (warmer zones in the US give 12 months, colder zones than ours suggest even less than 9 months). My vet knows that if we have a hard freeze and constant cold from Nov. through March I always have 2 left over. She prefers this in case we have an early warm/wet spell in March, then I always have a heart worm tab left over to start early. But, if your dog has a past history of heart worm, stick with the recommendation of 12 months of preventative.
As for intestinal parasites please read here:
http://www.sentinelpet.com/intestinal_parasites.html for Sentinel info, then see if your pup is still being treated for any of these or at risk of contracting any of these parasites. If no, IMO there is no reason to medicate 12 months. Why give our dogs unneeded meds? If yes, ask for something specifically for intestinal parasite treatment (also ask for a recommendation of an effective over the counter drug you can buy at a local pet store). My heart worm preventative runs about $6.00 a pill. I'm sure you can buy a de-wormer OTC much cheaper than that.
I'm in no way suggesting you ignore your vets recommendations. It's just important that you feel comfortable, informed, and empowered enough to question your vet because after all, you are the best advocate your pup has. A good vet will welcome your questions and suggestions.
Thank you for having your Halo on heart worm preventative. It's one thing I'm really passionate about. So many forgo it and the consequences are at best heartbreaking, at worst deadly.
Welcome to the Basenji Forums Halo and family:)