> I just worry about putting such strong chemicals on her so often
Me too! I don't ever use a spot on. Any odd tick gets a blast with the spray Frontline, so doesn't get used much at all.
Many vets recommend Drontal monthly, which I would never do, unless walking dogs in an area that was frequented by many dogs. I used to Drontal every 3 months, but, having left my 3 Collies for a year without worming, and then sending off faecal samples for the lab to assess - no worms were found - I now only do them annually.
However, it does depend on where your dogs go, and I realise that you have to take this into consideration.