Taking a hard stance on either side of this issue is silly. You're supposed to do both.

Do a dry run to see the encounter, then research the encounter to understand what you saw. Then you run it again with a better understanding of what's happening, and you end up understanding the fight even more each time you run it and confer with information about the fight.

As it pertain to new players in old content, for instance while you're leveling through DF, you can take time to explain and shout out queues during a fight if it seems that the newer players aren't picking up on it as it happens. It's the best way for them to actually learn and understand the fight, rather than letting them scrape by without a solid grasp of the encounter.

The correct motto is "research the fight, then learn by doing". You're not robbing anyone of a "first time" experience by telling them about a fight because it takes more than reading or watching a video of a fight to actually clear that fight.