If you are buying it brand new, it will come automatically updated with the latest iOS. If you are buying it used and it isnt updated you may or may not want to. As Toxic said, it takes up loads of room so if you only have an 8 GB you wont have a ton of room.
Thats my problem, I CANT update bc I dont have enough room. I dont mind but some apps require the update so I cant play/use everything.
Its up to you!