Hi Jennifer,
There are several valid reasons one might have for opening or retaining credit, and thus your questions can be answered several different ways. John is right on with respect to the credit scoring angle.
Another reason is to build a versatile and rewarding credit portfolio. This means different types of cards (Visa/MC, American Express, Discover, store cards) and different issuing agents (Citibank, Chase, credit unions) all offering good incentives for using their card (broad acceptance, attractive rewards, and low rates and fees).
Based on this standard, you're off to a great start. The Amex Blue Cash card and mtvU cards have excellent rewards programs, come from major issuers, and are widely accepted payment methods. And while you wouldn't want many store cards, up to a few can be worthwhile if you use them frequently.
If I were in your position, going forward I would not hesitate to open up to a couple of new cards per year that fit my needs well. Since Amex Blue Cash is already a "prime" card (and mtvU a prime student card), and your payment history is perfect, the only factor preventing you from getting most any card you wanted would be your household income figure. But that will change once you enter the full time work force.
Bottom line: you're off to a great start. Keep up the good habits, and don't hesitate to slowly build on your already strong credit credit base.