The best time to assess dag score in a winter rainfall region is in the spring time prior to hogget shearing when at least 20% of the flock have an average score of 2 to 3 (see picture). Consider culling all score 5 sheep prior to mating. Mark score 4 and score 3 ewes and note the number in the flock so that an assessment of how many of these could be culled from the breeding flock can be made.
Source: DAFWA website Managing non mulesed sheep, Breeding for flystrike resistance in sheep
In the summer dominant rainfall environment few sheep get severe dags, but those that do so in the fly season are at extreme risk. Therefore, it can be worthwhile to cull these sheep (same as culling sheep that actually get flystruck).
Generally, in environments where dag is sporadic or few sheep are affected, dag is better handled through management, rather than applying valuable selection pressure to dags, which could be used for other traits.