I have been giving this to Moose one pill a month- so far only 2 months & I really don't think
its such a smart idea- am thinking more chemicals in it so prob pretty bad.

Do you think there is any harm in just stopping & not giving the next dose Aug 1st?

If you live in a heart worm prone area then you have to give medication regularly,per instruction,to prevent the dog getting heart worms. In the current heat wave/high humidity ,the Mosquitos that carry the parasite are plentiful.You really risk a serious problem if you try to extend the dosing date.the monthly dose age ensures the dog is covered for the infection of the heartworm.parasite.
we use cardtotek plus here.Like you,I don't like giving chemicals to my dog but the alternatives would be risking an illness that is life threatening.
We  dose during the hotter parts of the year..march to November so the dog at least gets a break in the cooler months when there are no mosquito vectors to transmit the disease.
I've never had a problem in 22 years of living in a hot climate,so I'd say  trust the vet and follow his instruction on when to give this,relevant to your own climate .

(  Post continued.....I am thinking your question is concerning the idea of dosing at longer intervals...)
However......There is a school of thought that says..
"Heart Guard can be given every 60 days and only during mosquito season. Remember that these "preventatives" do not actually prevent heart worms from entering the dogs system, the kill the microfilia after administration of the drug. The drug is out of the system rather quickly so it's not that it's effective for 45-60 days, rather, if your dog becomes infected with heart worm microfilia, as long as the "preventative" is given within 60 days they will not mature. "

For most it is more convenient to give monthly, and, the drug companies will not cover heart worm treatment if the drug has not been given every month.
I remain with the monthly dosing group...if you forget a pill date you are at least covered for a few days over...if you dose every 60 days ,you risk missing the date and lose all cover.

Thanks :)