I use two factor authentication. On the page titled "Sign-in Code Verification" there is a small check box near the words "Trust this browser" that is by default checked off. Sometimes, I forget to uncheck it and as a result have to delete all my cookies. How about having it unchecked by default and users who want their browser to be trusted will check it off once and that will be it. This way users who don't want their browser to be trusted don't have to keep unchecking it.