Opinion is quite divided among chickenkeepers whether to worm regularly or only when signs are seen. You will have to decide for yourself.
Mites and lice, though, you should probably be inspecting for regularly -- not looking for signs of them affecting the birds, I mean, but looking for the mites/lice *themselves* on the birds and on the roost. Me, once a week I try to make a fairly closeup check of most of the birds' bottoms as they sit on the roost in the evening (will also let you know if there are diarrhea or other vent-area problems going on) and I try to pick a few of them up a couple times a week -- whichever chickens are handiest at that moment, I am not systematic about it -- and blow their feathers apart on the neck and base of neck and look real fast and closely for any wildlife in there. (e.t.a. - and then, after holding a bird for a couple minutes, I examine my hands and arms to see if anything is crawling on *me*, as northern fowl mites are easier to see that way than on the bird IME)
If I see anything at all, I dust all the birds thoroughly and their roosts/nestboxes/etc as appropriate (I use DE if there was just like one or two mites on one bird, sevin or rotenone dust if mites were more abundant or widespread). That is what works for me -- I've only had to do any dusting just a handful of times in the last several years -- but of course bazillions of other programs would work too
What does NOT work well is to fail to check regularly, because mites/lice are a heck of a lot easier to deal with when there are relatively few of them than when you've already got a galloping infestation.
Good luck, have fun,
Pat
