I used to do mine in late August/early September, a couple of weeks before the sale. Always dreaded putting in the yellow tag, because of how often they would not penetrate the ear, but bend sideways, leading to unnecessarily unhappy sheep and much bad language from Garmoran.
Last year I put them in at shearing time: it was easier to identify lambs because they were still sticking close to the ewes, the tags went in with very few problems and there were no infected ears, something I had not managed to achieve before despite much use of Dettol and antiseptic cream.
I wouldn't like to tag them earlier though, due to the high risk of tags getting torn out when caught in fences. Their sprayed-on number is usually still legible at shearing time.
I can't use them next year but if I discard them, then is some sheep inspector not going to look for those three sheep?
If the tags aren't listed as used in your Flock Record Book then I can't see why an inspector should look for them. Like Mowhaugh, I find I need a spare from time to time and I try to use a replacement tag from the same batch as the original.