I just loaded the first couple of Alaska cruises in May to check time. Skagway and Icy Straight Point have an all aboard time of 8:00pm and Ketchikan has an all aboard time of 6:45pm. So, for 2/3 ports, you'd miss the first dinner seating if you stayed out late.
I've only been on one Alaska cruise, but there was so much to do that I got back to the ship about 5-10 minutes before all aboard every day.