I've used K9 Advantix ever since the vet gave me the OK to begin applying it. Never had a bad reaction, so I guess I'm lucky. I originally started using it because the dog lives outside 24/7, and she had a nasty time with aggressive black flies and mosquitos. The Advantix does a killer job as a mosquito repellent, and seems to repel black flies too. As far as I know, it is the only brand that does which is why I chose it over the others. I continue using it through the winter, and have yet to find a tick on the dog since I started using it. Before I was pulling off 1-2 dog ticks a week, sometimes more. We did a hike down a poweline cut on the Tully Trail last summer, and I was particularly mindful of ticks after Dugan reported finding many on her dog after the same section of trail. I found about 4 on my pants and shirt after the half mile of 6 foot scrub, but none on the dog. This year we did the Lyme vaccine, and given the news reports I'm glad I did. With the costs of treating a pet for Lyme, I'll gladly pay a small price for prevention.
In regard to "pushing" these treatments to help profits, that may be the case for some vets, but ours was selling it at what I now find to be the cheapest prices around. They were cheaper than PetMeds by about $30 per year. However, my vet stopped carrying it due to lack of demand. I was able to get a prescription to get it through their supplier via mail order, but I still pay more than what I was getting it for over the counter. Maybe he was getting a kickback and stopped carrying it because they stopped paying him to do so. Who knows....
As for checking your dog - Up, down, and all around. They will hide anywhere. They seemed to really favor the underbelly when the dog was young, I would assume due to her astonishing lack of hair on her underside as a pup. They were very common just behind the "armpit" of her front legs.
Also don't forget to check your cats if they roam outside. Our strictly indoor cat escaped for all of five minutes, and she picked up a deer tick in her fur while I was chasing her across the front lawn and back into the house.
Smitty