There is a certain kind of weather that the flies just love, it's hard to describe but I can always tell, still evenings after showers on a warm, but not hot, day. It surprised me to learn how many types of blowfly there are, those around our area seem to be the small brown ones.
I never dip or use preventatives as we have such a small flock that unusual behaviour is easy to spot, the sheep start twitching and looking at the itchy spot as soon as any eggs hatch. I do keep a bottle of spot-on for established fly strike just in case, although it is almost out of date now. The worst sheep for fly strike we ever had never had a dirty bum, the flies used to lay on a waxy area of skin on her lower back. I used to refer to it as 'Hattie's diner' because the flies just made a bee line for it.
An old shepherd I know told me that heavy showers will sort out fly strike, has anyone else heard of this too? Can't say I would trust that to work, but it is interesting nonetheless.