I buy safeguard horse wormer paste. Get at your local farm store. And I heard worming them 2 times a year.. In spring and then in Fall or whatever you want.. And can't dewormed pregnant mothers and nursing babies.
I would only de-worm if your rabbits have worms. You can take a stool sample to the vet every 6 months to be checked - inexpensive and worth the piece of mind. (Tape test can be used to check for pinworm eggs).