My personal preference is not to put any pesticides on my furbutts. When we are walking in unknown areas or in woodsy and wet areas, I spray a light mist of lemon juice (from organic lemons)/water on them before going out. We have never had fleas (knock on wood) and ticks are something I check for on a daily basis but don't treat them with any chemicals.
I believe in the many uses of essential oils and have been incorporating them into our lifestyle. But you do need to do your research on them as far as using for pet purposes. Lavender oil is one I have used on the dogs (diluted) and never had issues.. Only true Organic pure essential oils, though, so check it out.] Again, do your research first
Some oils are toxic to cats.
Here is a couple good links for info on natural alternatives:
Natural, Non-toxic Flea Control: Safe products and solutions | Eartheasy.com 5 Solutions for Natural Flea Control | Care2 Healthy Living http://www.joys-of-lavender.com/flea-control.html