The only time I've had steam not work in offline mode was when it needed an update. The update was downloaded and just waiting to apply. When I lost my internet connection and restarted steam, it couldn't verify the steam update so got stuck.
Otherwise, I've never had issues. I've been gaming and lost my connection, so I flip on offline if I want to change games, steam restarts, and away I go. There have also been times where I launch steam for the first time, it doesn't detect a connection, and asks if I want to start in offline mode. I do so, and no issues.
Though I believe that sometime prior, for every fresh install, I had toggled offline/online once while still with an internet connection. I also haven't really tried launching a bunch of random games for an initial launch while offline as the outage is only a short duration or I planned ahead. I would imagine that a game would have to have been launched at least once prior to going offline. I also haven't been without an internet connection for more than a week yet also having a PC with steam installed, so I have no idea if it needs to reconnect periodically like SC2 does.