It is not essentail but if you have one with you why not but not necessarily every night. You will have more casual nights than informal but see what the others are doing and if you feel that you should it is up to you. I always say dress in what you feel comfortable in on these type of nights and dressed on the formal and you won't go wrong.
You will find there are more formal nights than you probably had on OV. We were on there a couple of years ago, and whilst Ventura is fairly casual it's not another OV. Casual nights shirt and trousers, semi formal same but a jacket if possible, but they are not rigid. Formal nights really go to town, DJ or dark suit for men, and long, cocktail or OTT trouser suit for the girls. Enjoy!