Ivermectin is used in the "slow kill" method to treat dogs that have heartworms. So no, you do not have to be sure your dog is heartworm negative before starting the preventative.
Some vet techs/vet receptionists will tell you that you can kill your dog if you give heartworm preventative without having a heartworm test first. This is not true. In the 70s and 80s we used a medication called Filaribits, and if the dog was not heartworm negative before prevention was given he could have problems. This is not the case with ivermectin.
The ivermectin only needs to be given about every 45 days. The reason that companies recommend once a month is so that people do not forget. (Always give heartworm preventative the first of the month, etc.) If they tell people every 45 days, dogs are more likely to miss a dose, be given medication late, etc.
So yes, it is okay to give it once a month. Try the first of each month.