Back in the good old days (not that long ago, but you know what I mean), I did rotational deworming, administering a different dewormer every 6-8 weeks throughout the year. Did it work? Maybe, but it also probably contributed to parasite resistance to dewormers that were being used.
The new thinking on deworming is to do periodic fecal testing, and focus deworming efforts on those horses that show up as high shedders - this doesn't necessarily mean they're unwell due to the parasite load they carry, but they tend to be the ones that can spread worms in the environment. And good manure management - manure removal and at the very least harrowing so manure is broken up - can make a big difference. I followed this protocol at my old barn (very small, and with very good manure management practices), with very good results.
In the past several years, since at my current barn my horses are turned out in large herds - the mare herd that Dawn is in has about a dozen horses and the gelding herd that Pie and Red are in has close to 20 geldings - and since my barn practices zero manure management in pens and pastures and has no worming policy for boarders, I had reverted to something approaching my old practices. I did spring and fall deworming, and also did daily Strongid.
Well, I've been proved wrong. I recently fecal tested Red, and he came back with a heavy load of strongyles. I've just completed treating him with a Panacur 5-day Powerpac at the recommendation of my vet (Red and I are both glad that's over with), and will do a fecal retest in about two weeks. I also just tested Pie, and he came back as a zero - effectively no worm load. They are turned out in the same pasture everyday, and follow exactly the same feeding and deworming protocol. Pie appears to have an inherent resistance to parasites, and Red does not. So the same program doesn't work for both of them.
My theory that a regular program was required to, and would, protect all my horses at the new barn proved wrong - I'm going back to fecal testing and treating my horses as they require.
Old dog, new tricks . . .