I only ever turn a forum offline when doing an update. I get annoyed by people when the disable their board for a week to add a new style. It is quite silly to be honest as any activity they had will be lost.
My forum doesn't go "offline", just in maintenance mode where only staff can login. I only put it in mm when I'm fixing big codes, editing the default skin, or rearranging the boards/categories.
Thee are times when a forum must be taken offline. However, keep the downtime as short as possible, so that you don't lose members. Give your members notice that you will be going offline and post a note on your site, during the down time, to say when you expect to be back online.