I keep between 600 & 1000 shedding ewes (depending on land availability) and very rarely treat adult sheep with a fly strike preventative
Sometimes if i am going away for a couple of weeks i will treat lambs if they haven't shed well
I graze land on top of Salisbury Plain ---- low risk due to wind/low humidity as well as places like Stourhead , Wilts which has many lakes and is surrounded by woodland---hi risk and hi humidity
When we kept Wiltshire Horns we would sometimes experience head fly problems and would have to treat with a dab of something on the head
It's about risk assessment , management and genetics ---any sheep that ever gets strike gets culled in my Charollais flock and i am convinced that this reduces risk