How to change initial ranking of category groupings?

When you sign up, you're asked to nominate three category groupings that are productive and three that are not. Obviously, if you're not fully aware of how RT works in detail you might not make the best choices here. (Also, in my case, my work isn't completely conventional and so doesn't fit neatly in the category groupings provided.) My questions therefore: How do I get back to that screen to change what I nominated? And do I need even need to (i.e. do they have any impact on the tracking/reporting that goes on or are they merely for user-survey purposes)?