You can compensate for delays in the video output that may be introduced by external decoders and certain types of displays (such as projectors or plasma screens).
The amount of delay introduced varies based on your system and the type of video peripheral you are using. When using an Avid peripheral with QuickTime, 18 quarter-frames is a good starting point, but you will need to verify the precise setting for your system. (Third-party devices are available to help you measure this delay.
Once this value is set, it should not need to be updated unless you change components in your video monitoring chain (such as projectors or plasma screens).