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 2009 Carnival Conquest 2010
Grand Princess 2010 Island Princess 2011
Grand Princess 2011 Carnival Magic 2012
Carnival Dream 2012 Island Princess 2013
Carnival Magic 2013 Carnival Legend 2014
Carnival Magic 2015 Coral Princess 2015