Are you talking about the garment bag that is like a suitcase or the plastic/vinyl ones that hold one or two items and the hangers stick out the top? If it is the garment bag that comes with a set of luggage and the hangers are completely enclosed, then yes go ahead and have the porters handle them.
If they are the garment covers or the type that just cover the clothes and have the hanger hooks stick out for hanging, then carry them aboard yourself. The luggage is handled similar to the airports, all piled in a cart and brought aboard. I would be afraid of something happening to my bag or my clothes getting crushed.
It is a garment bag that zips from top to bottom with the hanger sticking out, but it did not come with our luggage. I hate packing it in luggage.
I would carry one like that on as it does not really give much support and the hangers sticking out can catch on someone else's luggage. Also if your bag is at the bottom of the stack with others heavy luggage on top, your clothes will be completely wrinkled. The luggage is loaded on the ship by longshoremen.