You have already received answers, both pro & con, that are evidently not satisfactory. So I'll provide more detail on what works for my operation (not yours, not my neighbors).
I do not have a large herd; average 50 cow/calf pairs. I see my cattle every day; multiple times during calving. My tagging system is a cluster only I understand with different colored tags, a numbering/identification based on lineage & number of calves per dam, right ear = bulls, left ear = heifers, etc.
I don't vaccinate my cows with Sour Bos prior to calving (used to but, long story), so each calf gets a First Defense bolus, preferably within 12 hours of birth. Never had a problem with pneumonia until a few years ago, so added Inforce 3 at birth when I tag & administer the bolus.
When I go out to take inventory, I can easily identify the calves, based on their tag and check them off the list. Calves are not always with their dam - especially when they get older. If a calf is sick and I need to get it to the barn, I don't have to wander around all day, waiting for it's mama to start looking for it & bellowing.
Tagging at birth works for me because I keep diligent records, which proves beneficial when deciding which calves to retain and which cows to cull.
A phrase I use a lot is "You do you".