It depends on the content and how new the content is. If it's the first few days of new content and people are coming there for the first time, I consider explaining the mechanics as spoiling the fights - when explanations aren't asked for. It's really horrible entering the new normal Alexander or 24 person instance on day 1 and there being people already expecting others to listen and follow the strategies they're calling. When the content isn't completely new anymore I usually ask if the newbies want explanations or not. And even if they do, I'll do my best to not actually spoil the fights and instead just warn them about the stuff that they might wipe everyone else with.

Personally, I'll enter all fights blind, but most of the time I'll do that with full premades so it doesn't even affect anyone else. For 24 person stuff I'll turn off the alliance chat if there are people spoiling the fights.