It's a good idea to clip all around the back end, the area which can potentially get filthy, in advance of the shearer coming. Often you have to remove this wool from the fleece after shearing anyway. If you are worried about flies, then spray just the cleared area, not the fleece, but of course you would then have to do the whole sheep after shearing. Definitely spray your lambs now - they are more susceptible to flies, and what's a small area on an adult sheep is much bigger on a lamb.
Don't feel too bad about the tiny patch of strike on your tup - you've found and treated it very promptly. For the rest of the flock, watch them carefully in case flies get somewhere not clipped before your shearer gets there.