Should "toggle buttons" show what they do or the system status? [duplicate]

Possible Duplicate:
Should a toggle button show its current state or the state to which it will change?

I find inconsistency in "toggle buttons" (I just invented the name now. See screenshot to know what I mean). I see GUI controls that may show the action that will be done or the system status. This is confusing, because I don't know the convention.

I feel that the more natural way of rendering toggle buttons would be to show what they do instead of system status, just like multimedia buttons (play, pause, next, etc). For example, if media is playing, the "play" button is rendered as pressed, just like on cassette players.

What's the best practice? Should they show those buttons show system status or should they display what the buttons do? Are there cases when it's better to show system status or vice-versa?