Worming sheep is a huge topic, and as we move into the future of the industry a huge can of worms.
[sorry. gets coat...]
I agree that taking fecal worm counts is the logical way to go. Don't just blanket worm 3-5 times a year because of what the calendar says. However the interpretation of the tests must involve you, not just the vet selling the expensive wormer drench. Be aware!
A sheep specialist vet is the one to listen to, sadly a rare breed here in Canada.
Sadly, worm burdens will likely be worse for smallholders on a small acreage who do not make their own hay and where sheep have been kept for many years.
One of our worst worms here is the Barber Pole worm. This sucks blood from its host. It is relatively easy to kill and the best way to see if it is present in your flock is to take random or poorer sheep and roll down the membrane under the eye. If the flesh there is pinkish red that is good but if it is white or grey then the sheep is anemic. A good infestation of these worms will drink a lot of blood and the animal will eventually die.
But the real problem is the advent of Superworms. We have 3 main 'families' of wormer drench chemical, whites, clears, and yellows. Most people now try to use a different 'family' of wormer each time they dose, and not stick with the same one. But despite this there are worms that cannot now be killed by these chemicals. A small proportion true, but if you kill all of the susceptible worms and just leave the the whole gutspace vacant for the Superworms they quickly repopulate the whole gut. Now you have 100% of a wormload that you cannot treat.
All sheep and animals, even people, have worms, organisms learn to tolerate a certain burden, but it is better IMHO to worm less often, and on a fire brigade principle, and to live with the ones that you know you can kill if you have to.