I just use the small button 'secondary' tags when the calf is born, for accurate identification. I am keeping weight gain records, so need to handle them in the first 24 hours after birth and tagging them at the same time is sensible. I spray the eartag point with Iodine to prevent infection. Fortunately we aren't inspected at such an early age, but all the eartags need to be in before the animal leaves the property or the herd needs to be Tb tested.
I've just had to order another set of eartags for our bull too. He got into the bush and managed to pull his original tag out. We now have to have two plastic eartags in each animal by law. The brass Herd book tag is optional. We used to be able to have just the brass tag and one other plastic tag, but the rules were changed last year. We can however choose the size of eartags, to a certain extent. I usually go for the largest sized primary tag for the heifers I'm keeping for breeding, as its a lot easier for me to tell which aniaml is being served from accross a valley - using my binoculars. The steers just get the economy sized set of tags. I know eartags are a nuisance, but we do need them to keep our meat exports safe.