It's hard to predict when you might need lip sync (or AV sync) control. It all depends on whether a component somewhere in the signal path will introduce video processing that creates a delay in the video signal. An HD cable box may very possibly do this if you ask it to upconvert SD content or scale all HD content to a single output resolution. The 980H does have a little AV sync control, but it is slight and it is not available when using digital output (since it doesn't manipulate the audio signal when using raw digital output).

An HDMI-to-DVI adapter is nothing more than an HDMI cable with a somewhat different plug on the other end, so no signal manipulation takes place there is no picture quality degradation caused by using one.