If there are going to be four of you in a cabin, I'd argue against it. It certainly will fit under the bed, but that space is best used for empty suitcases, and with four of you there will probably be a lot of those. One hint is to plan your suitcase sizes so you can put one inside another before putting them under the bed. If you can determine ahead of time that you can fit the guitar along with all the suitcases under a queen size bed, well, then maybe you can give in. Of course, if you're very nice to your cabin steward he'll probably take some of your suitcases off your hands if you really run into a bind.
Also keep in mind your airline weight and piece limits. I certainly wouldn't pay overweight/oversize charges to the cruise and back.