You could just take pooh samples over 3 days and have the vet check for worms, if there are no worms present you don?t need to de-worm. The worming treatment isn?t preventative anyway but if you have a pooh test done regularly you?re on the safe side without adminstering chemicals unnecessarily.
I add a little dessicated coconut to Humphrey?s raw diet, (you can also use try garlic, raw grated carrot, ground pumkin seeds, there are lots of natural things you can add which parasites aren?t very fond of) and since he?s been on a raw diet we?ve had no problems with worms or digestive tract parasites at all.
The decision re parasite treatment is, as you can read in a lot of threads on here, a very personal one and everyone has to make their own minds up regarding the risks but it is my personal opinion that the risks from the treatments are a lot higher than most vets will admit to so I only use them if absolutely necessary.