Just dag her backend, and keep your eyes peeled until shearing. Watch lambs too - mucky bums are the most likely place for maggots, so avoiding those and/or treating straight away is the most important.
Sheep also get flystrike in their feet, if they step into muck, boys on the underside of their belly too.
You don't want to treat with anything chemical (as a preventative that is) now, as shearer will not want to be exposed to those chemicals. Withdrawal on them is quite long, certainly more than a month.
You can get a flystrike spray (from the vets) which is ideal size to carry in your pocket when you are inspecting your sheep, and you can then treat in the field straight away. Kills maggots and also soothes the skin. Good stuff.