No, you do not need to use the auto-fill function to start creating your product. After you choose your theme, on the photo upload screen, instead of uploading photos, go to the upper right corner and click CUSTOMIZE MY BOOK and continue to the next screen.
Once you are in the designer, you can add photos at any time by going to the Photo section on the menu.
To learn how to skip auto-fill and get into the designer first before uploading photos, follow the steps in this video: