Products such as Vetrazin, Crovect and especially Clik are toxic to varying degrees - otherwise they wouldn't deal with the flies. Many people are badly affected by contact with them - this includes those who apply the product, those who shear the sheep and those who handle the fleeces, such as Wool Board workers, and home craft workers. Judges at shows can also have problems if all the sheep in full fleece they judge have been treated with such products. When fleeces are being processed, the water used goes back into the environment so has to be treated to remove these products, at expense to the processors.
If it is a professional shearer shearing your sheep, he or she could be exposed to a huge dose of these chemicals over the course of the shearing season, as could the Wool Board worker - they are not handling just one animal.
So to avoid harm to these people and to the environment it is best not to apply the products closer than 3 months before shearing, in the hope that they will be de-natured by then. If you have to apply them to a struck animal, then destroy the fleece once shorn - but that doesn't help the shearer. We shear our own sheep so we can decide for ourselves what to do. It is worth discussing this with your shearer as he or she might not have a problem with the chemicals, or might have a big problem with them. It is certainly something worth thinking about.