Everyone does it different, some just blow them out, some just pump antifreeze through, some do both and some drain after they put it in. Sometimes the issue is water stays or gathers in a low spot or the little nooks and cranny's of a faucet or something so sometimes blow it out is not quite enough. Adding antifreeze can be easier went the lines are empty as it does not have to push the water out. Once the antifreeze is in and has replaced the water, what is the difference if you leave it there or drain it. Whatever has worked for people, they just keep doing the same until there is an issue and then they will change it up.