Well, here's how I do it:
1. Reading - not a context, but a list I have in my memo app. in my Palm
2. Commenting - too general for me. I need NA's to be more granular. If I have to get back to someone on something, I have the VERY NEXT PHYSICAL action defined and in the appropriate context. If it's more than one, there is a project as well.
3. Brain dead - I don't really have anything here... if it's something like, "refill the stapler"... I just do it. (in moments where I don't have any energy, I'll check the stapler, paper clips, etc.).
Anyway, if the above works for you then great! There really is no "right and wrong" thing to do. There are only systems, and you have to make yours work for YOU.