+1 to always ask for auth code. I suppose that when you allow(ed) a network…Its a bunch of IPs from this network that are able to log in without asking for a code, not only the one you have/had and permitted. So even If one more person can access my account without giving auth code for me is not secure.
So I think the solution to always give the auth code is the best. If anyone finds it frustrating then do not activate it. Good Security always need more time and credentials. I am aware that I need to sacrifice time and battery (mobile phone) to be more secure and I want that.