Asking before fight is fine, then you know what is going to happen. Standard raiding for new players starts with asking them "Everyone know the tactics?"Asking halfway through getting wiped, thats a different matter. :lol
If you play a tank, you have 2 main jobs. Marking targets for the group and explaining tactics. You do this because you actually know what you're doing and it makes it easier for a tank (who controls the pace of the encounter) to mark things up and explain areas that might need the groups attention.