Yes, you do need the stickers. All Linux distros have a "sticker" that are required to run, the only difference is that they are saved as images on the distro itself... hidden. That means that each time you install Ubuntu, you are actually installing the sticker.
Unfortunately, the sticker requires something, sometimes the sticker will tell you to do things in return for the free operating system. The sticker can also leave the install at will and travel into other things, such as a dog. The sticker inside the dog will then tell you to look for hidden messages on the covers of Deitel and Deitel programming guides, at least that is what my sticker told me. I have yet to decipher the sticker's strange and beautiful codes that it left for me on the cover of my Java: How to Program book, but as soon as I do... I will try to perform the task.
Also, I suggest against installing it in virtual machines, as the stickers are cloned and soon the stickers will run out of beings near your home to possess and may start going into dust mites, which easily get on your clothing and are harder to hunt down and purge should you chose to uninstall the linux distro.
In order to uninstall the distro, you must find and cleanse the thing that the sticker possesses. The voices of the stickers get numerous quickly if you install many times.
The box stickers can sometimes trap the real sticker, but it is a crapshoot.