I test every dog, every year (and I’ve got 5 now), even though my dogs are on preventative year round and I’m very good/obsessive about not missing a dose.
Just finished up the 8 weeks of crate rest for the split dose of Immiticide protocol for heartworm treatment on what was supposed to have been a foster dog (I failed, he’s family now) and I don’t EVER want to go through that again. The slow kill protocol (wherein one simply gives the infected dog ivermectin to kill the microfilarae and then waits for the adult worms that are already infecting the dog to die off naturally) isn’t something I’m willing to do.
OP - you might consider the cost of testing vs. the cost of treatment. Testing is cheap and safe - treatment is expensive and can be risky. And as for having a dog who “seems healthy”? Failed Foster Dog is young and ridiculously active - prior to treatment, I never heard him so much as cough - but he tested positive. 8 weeks of crate rest is no fun (for dog or owner) and honestly, even though we did a blood smear and saw no microfilarae at the end of that time, there’s a part of me that will worry right up until we get our six-months clear test in December.
As for the Lyme test - it’s just part of the snap test. Doesn’t cost any extra, and I don’t treat unless the dog is symptomatic. (And given the cost of doxy at the moment, I would hope most vets would feel the same way…)