LOL! I went around and around, mentally, about that 'keep him quiet' admonition. The stuff on websites is very general, and it's directed at ALL dogs who have heartworm treatment, regardless of the dog's condition, the level of infestation, and a bunch of other variables. I do know, in rescue, a number of dogs who were treated for heartworm, and were largely crated after the treatment; but they had much more severe cases, and they were, well, rescues -- in transit to another foster home, not part of the resident dog population where they were, etc.
So I figured I'd go for the reason behind the 'keep them quiet' instruction: the worms, once they die off, are breaking down in the body and being reabsorbed. Pieces of worm can travel to the lungs, where any ratcheting up of activity level can cause worm fragments to get trapped and form a pulmonary embolism. (This is all SO GROSS!) Okay, so I want to not get my dog's heart rate up, or respiration rate. I can do that, that's much more specific than 'keep the dog quiet.' And I want to make sure my dog has lots of down time, rest time. So that's what we're doing. I also know to look for ANY signs of respiratory or pulmonary problems, from coughing to sneezing to nosebleeds, and call the vet clinic ASAP on seeing them. So far, although the online info says coughing is not unusual in a dog at this stage, Rowley's done none of those things.
This is pretty much a learn-as-I-go process, but it is helpful to sort out the reasons behind the protocols, to me. I don't follow instructions just because instructions are given, and I never have.