Great question.
One key factor to the credit score is age of credit, so it's important to keep your cards open (bureaus equate, I think, age with stability).
And, ideally, I think, you'd keep small balances on a bunch (that you pay off each month, or keep low enough not to incur silly interest) as a way of demonstrating your responsibility.
Which is to say, keep 'em open, use them to buy movie tickets or whatever, and I suspect both (keeping them open and moderately active in a responsible way) will aid in score improvement.
Thoughts?
