I tend to be a last minute shopper and with cruises it's sometimes good. But it all depends on what you want. For me, I don't care where I sleep or shower and I sleep like a rock, once I fall to sleep so... my thinking is.. if the bed is soft and the water is hot, give me that cabin.
My experience with booking last minute.. 5 Night Cruise $249PP and 3 Night Cruise $199PP.... However; when I do that, I usually end up paying more for a flight... so.. I guess it all evens out..