If you book a guarantee the only way they can "bump" you is up! You might get the worst cabin in the category you are guaranteed, but it's more likely that you'll be "bumped" up. (This depends on how early you book, generally.)
Never book a guarantee in any category you won't be happy with!
"A truly happy person is one who can enjoy the scenery on a detour."
Golden Princess 2009Carnival Conquest&
Grand Princess 2010Island Princess&
Grand Princess 2011Carnival Magic&
Carnival Dream 2012Island Princess&
Carnival Magic 2013Carnival Legend 2014&
Carnival Magic & Coral Princess 2015
Carnival Liberty 2016