It depends. I'd like to tie mine with a vise, but I usually forget to do it before I go. If I'm on the water and I remember to bring the "magic thread", then I'll tie it and trim it there. I usually run the hook through the ball, and slide it up. If I have an egg loop tied on, I can secure the yarn and the roe with the loop. Sometimes I just run the yarn with some scent on it.
Hands down, I only like to do mine in a vise at home directly onto the hook. They're quick and easy and I can tie up a 46 pack of hooks in about an hour. I don't use bait often, and never with a yarnball so an egg loop isn't needed.