Depends in most cases. You just have to strike while the iron is hot.
For example -- videos for WFL and LFN (TGIF) definitely benefited from digital sales. WFL went to #1 on the charts after the video release, and LFN (TGIF) shot from #31 to #4 because of its sales. They were singles that were just starting off and steadily gaining in airplay and sales.
But most times, some artists/record labels don't strike the iron is hot -- i.e. waiting until a song has peaked (when it clearly could have gone on to a better peak). For example, if a song is stuck at #11-13 on iTunes for a while, a video could send the song into the top ten on iTunes, and most songs usually receive a hefty boost in sales when entering the top ten. Maybe record labels/artists aren't too aware of this like we are (since we usually study the charts more than they do

), but I'm sure if they knew that, they'd take advantage of it.