We started banding at birth and haven't looked back. We band, give antitoxin and tag within the first 12-24 hours. No problems as mom seems to help take care of keeping it clean etc. We also notice no irritation or pain on the calfs part this young. Seems so young the nerve aren't quite as active yet.
The ranch I work for stopped knife cutting years ago and bands everything now. Every month we run the calves threw. Band the calves that aren't banded or were missed in the previous round up. The older calves seem irritated but immediately go back to nursing. Occasionally we get one that lays around for a few hours swishing its tail. Then it's back to normal.
Takes two of us to run it smoothly. We keep 3 sizes of bands and banders on hand. Usually only need the smalls though. At this time we sort animals that are leaving for other pastures, weaning or headed for auction.
Hope this helps

Remember to check and recheck for 2!!! You wouldn't believe how many times you swear you have 2, take the tool off, feel and one disappeared! Lol! Stay above the actual testicles but below the little teats.