Under what circumstances should a site be placed into maintenance mode
In my experience working on some very high profile D7 installations, maintenance mode is always required during updates. The steps you took to perform your update are essentially the same as I've experienced. The reason for using maintenance mode is to essentially lock out users from making database changes that might corrupt the DB when update.php(drush updb) is run. I can't think of any good reason why you would do a production update without maintenance mode, unless you like playing Russian Roulette with a production DB. I also hope you're doing the updates during non-business hours or during a scheduled maintenance time.