It wasn't practical (for me) to develop both of the above methods into my application, so I took a leap of faith and allowed the user to choose their own IDs. I quickly found that this complicated development so much that it would have added weeks to my development time, and resulted in much more complex and slow DB queries. So, early on in the project I went back and made it so that I just assign IDs for all created resources.
Life is simple now.
Other popular APIs that I looked at, such as the Instagram API, also assign IDs to certain created resources, which is especially important if you have millions of users who can interact with each-other's resources.