I don't use chemicals on my adults, only on susceptible lambs, and usually only if I will be away myself, to reduce the anxiety of the sheep-carers remaining!
Shetlands are less prone to flystrike than many breeds, but any sheep can get struck.
Keeping rumps and undercarriages clean (through dagging, keeping them healthy and untroubled by worm burden, and by not having to lie on very soiled ground) is another important element of prevention. Also being on top of any foot infections, maggots can start in the foot and spread to the body when the sheep is lying down.
Once a day checking isn't enough if you're not using chemicals. Twice a day minimum. You need to be spotting any maggots starting to nibble, you can't risk them having 8+ hours feeding. (Once they start to feed, chemicals attract other flies to lay eggs, and the population explodes very quickly.)
We do get a strike more years than not, but almost always get it when it's just one to four maggots and usually before they've really broken the skin. It's a lot of work and stress (for us, not so much the sheep!), but as long as it's us doing the work and having the stress, and not the sheep being eaten alive, I don't mind! If we weren't as good at spotting it and dealing with it, I'd spray the lambs once I judged them at risk, rather than have to deal with a severely struck sheep.
We had strike on the end of a tail in October one year. I'd been away, went to have a look at the sheep. I noticed a dirty tail and thought I'd better get that dagged tomorrow, then saw the lamb jump and decided to dag it right now. 4 maggots, just breaking the skin.