My husband and I just returned from our Alaskan cruise to celebrate my 30th Birthday. The entire experience was breathtaking and truly something you must experience for yourself.
We chose the Northbound cruise because I liked that it stopped at all of the ports I wanted to see, and I definitely would recommend going this way. On the southbound, you pass many of the sights in the early am, where was with northbound we passed them in the early evening. Because of this, I would also recommend choosing the late seating for dinner. More often than not we would be passing wildlife, glaciers, etc. and it would be right in the middle of the early seating for dinner. With the late seating you are also able to linger more and enjoy your meal.
I also purchased Fodor's Alaska Ports of Call 2008 and highly recommend the book. They give great tips on places to visit in each port, where to shop (that is not a tourist shop), where to eat and get a drink.