I can only answer from my perspective....
Just ask. People will tell you what they need. If you are really looking to help someone on the forum - and that's your motivation for doing something - ask them what they need for you to be helpful.
I think to do otherwise would be sort of paternalistic, you know? If you just ask, most people will let you know. If they need help evaluating batches, you can direct them to some of our help sections. Or give them a few guidelines - based on your experience.
If they need something else - they'll tell you. As I wrote, that first batch I worked on (that made significant money - for me) - I remember just who posted it and when. It changed my whole day (and week). It was really just a godsend -. I pm'd the person, because several people were angry that it had been posted at all
. It was my first big break on mturk and I learned a lot from that.
But as for a general rule. I don't think you can make one. I don't know what anyone needs. I just try to help. If I'm not sure, then I ask.