I have DirectX 9.0c (December). I've noticed that there have been "updates" to 9.0c evry few months or so. There is currently 9.0c (August). Is it recommended to update it every time an updated version comes available?
I check for updates routinely for all my software. It's just good practice.
Probably you can get by without updates for a while. If you let them lapse for too long, i.e. more than a year, then don't be surprised if you have problems with your video apps.
DirectX 9.0c is rock solid, and generally can be relied upon... If there is a new update, it will be labeled, 9.0d, 9.0f... etc... but all the work being done these days is with DirectX 10 which probably won't even work on your machine.
It might help you to know what Direct X is: From Wiki:
DirectX (originally called "Game SDK") is a collection of APIs for easily handling tasks related to game programming on Microsoft Windows. It is most widely used in the development of video and computer games for Windows. The DirectX SDK is available free from Microsoft. The DirectX runtime was originally redistributed by computer game developers along with their games, but later it was included in Windows. DirectX 9.0c is the latest version of DirectX. ...
This should help you determine if you need any frequent updates.