When you update WordPress and update plugins, it does automatically put the site in maintenance mode for a short period of time - if you refresh the front-end site when an update is installing you will see the message regarding maintenance mode.
Aside from that, you don't need to worry about it when you are installing plugins on the site or when you are adding or updating content.
I suppose the only exception is if you have a very high traffic site and you don't want anything to disturb your visitors and your conversion rates. In this case you could maintain a development copy of the site for testing changes and then schedule in those changes to the live site out of ours or at low traffic points.