I wait till they are up and sucking and being mothered.
If they are born in the am (as they often are outside) I might wait till later that aft/evening to mark em up. I do it in one fell swoop - spray, tag etc and then dont touch them again for at least a month.
If the lamb looks weak, I wait longer (no point tagging a dead un) and might give it thermovite. If is pissing down and everyone is cuddled up with mum, I leave em be.
Once you get to about day2/3 you have no chance of catching them. If that happens, you'll have to mark them up at first gather - hopefully the ewes number will still be showing and you can pressure them to find mum. Live lambs are better than dead, correctly labelled ones, IMO.
If you aren't performance recording, you can just leave them be entireley and just tag the lot when you heptavac.