It's a really different equation depending on your location and climate, lifestyle, breed or type of sheep, and many other factors. I wouldn't discourage anyone from putting the welfare of their livestock first; untreated flystrike is a dreadful thing. People who are experienced in spotting early signs, and who are able to keep a very close eye on their sheep (being able to see them all properly at least twice a day in the fly season, and catch them and treat them immediately if any show any sign of strike), and whose locale and microclimate and/or type of sheep mitigate against strike may very well find they can do without the chemicals. Novice owners, smallholders with day jobs, and people in warmer, moister, less windy places with fleece (not shedding) sheep, may well find that the chemicals are a necessary evil.