Contributions of intellectual property are a form of capital contribution, and as such are not required. Nevertheless, it can be a good idea to assign all existing IP relevant to the business at the time you form your LLC, to ensure that the LLC has all necessary IP rights in order to conduct its business.
If you fail to assign relevant IP at the time of formation, or if you later determine that the LLC no longer needs certain IP that was assigned, the tax treatment of an LLC allows you to easily transfer IP into or out of the LLC as needed. You should work with legal counsel if you plan to assign IP to or from the LLC.