One of my maxims is, "When in doubt, serve the needs of the student in front of you." For example, you are grading papers during a planning period and a stduent walks in for help. Do you ask him to come back at lunch so you can finish or do you stop and address his question?
There are many other scenaarios that can be mentioned that involve a potential conflict between what you want to do (even if you are trying to help) and what a student needs you to do. Remember, serve the student in front of you and help her based on her needs, not yours.