I've seen some horrific photos of poor animals with huge chemical burns from flea and tick treatments. I won't use them on The Princess, I use natural rose and geranium oil, rubbed inside her collar, and she's never had fleas. She's picked up the odd tick in the woods, but the tick picker has quickly dealt with them.
We had a cat with a flea allergy which was exacerbated by a pollen allergy in summer - poor thing looked permanently scraggy even with treatment! As far as I can tell (and I look all the time) the dog has never had fleas so we have never treated her.
Most of the flea treatments seem to contain the same chemicals as head lice stuff - in fact one vet I know used to use them on her kids