Provided that you've
1) downloaded the latest version of the game from steam
2) run the game at least once in online mode
3) set your account so that your password is saved and it auto turns on/logs in at startup
Then you shouldn't experience any problems with going into offline mode provided that you remain on the same PC configuration. Changing the PC configuration might spark steam guard (which you should have enabled) but then you only need access to email and a very short time online to fix that issue.
4) successfully exited the client after putting it into offline mode
If the steam process gets killed (like you turned off your computer before closing it, or you had a power outage, etc) then the offline mode won't work.