I have a small Chihuahua that I rescued off the interstate last year.
I took him to a garden store that had a visiting vet and got him his shots.

So we took the dog on a camping trip and the wife heard that we should get him some worm medication.
We did camp in area high up on an alpine lake with mosquitos...I ended up spraying the dog with OFF in a few areas that were hard for him to lick and gave him a bath when he got home.

Started reading up on heart worm medication and it turns out the problem may be somewhat overblown?
I can get the same medication (ivermectin) for sheep or a horse...but I need a prescription for a dog?
I'm wondering if it's something I should really worry about it at all...or perhaps treat him once during the summer months.

http://terriermandotcom.blogspot.com/2008/05/billion-dollar-heartworm-scam.html

Your risk in ID should be very low. The highest risk is in the Southeast and Atlantic states up to around DC.

Google image search "heartworm risk" to see some maps of risk across the US.

ETA: Some breeds, like collies, are sensitive to ivermectin, so look yours up before giving any! Also be aware that the doses for dogs are different than for other animals. Too much is a bad thing.  

I generally don't give my dog medicine for it but I picked up a supply for hunting season so he gets it for a few months. He's an indoor dog so I don't worry about it.

The reason for the prescription is the liability.

IF the dog has a heavy infestation of HW, Ivermectin killing the things is likely to cause no end of problems.

Testing the pooch before preventative treatment, negates the liability of the manufacturer.

Ours are on Heart guard year round now. Our old lab got heartworm in winter, back when we used to only treat April through October.
The treatment for an infected dog is a crapload more expensive than preventatives and an annual checkup.