Some people worm as a preventative measure (usually twice a year), while others don't. I haven't wormed my flock until this year. A benefit of worming regularly is to prevent any problems, but doing so can also create worms that are resistant to wormers. I would play it by ear and not worm your flock unless you start seeing problems (lethargy, weight loss, worms in droppings, etc.) or get it confirmed by a fecal test.
As for other preventative care, there isn't much to do. Just make sure to keep their coop clean and not stress them out too much. Also, check them often for mites/lice and treat those if necessary. Mites will appear as black or red dots on a bird's skin or feathers, and lice will look like yellowish blobs on the skin or small white eggs on the base of the feathers. Treat by dusting your flock with 5% Sevin powder or poultry dust, or by spraying them with another lice/mite spray. Make sure to spray the coop down with vinegar or liquid Sevin while treating for mites, too, as mites like to hide in crevices and re-infest birds at night.
