The ones I’ve seen are Heartgard, Iverhart, and Interceptor. Does it really matter which one I use? Are they all equally effective?

Also, will my dog need to go to the vet before I start her on heartworm medicine? I would rather order from 1-800-PetMeds than the vet, but I wasn’t sure if they’ll need to test her for heartworms before I start giving it to her.