Theoretically speaking, you don't need to make backups of your contents if you plan to root your phone, but you should create backups on a weekly/monthly basis or when you're about to use tools that modify system partitions. In those cases, create a backup and then root your phone, just in case something goes bad with the rooting procedure.
As a side note, I have rooted all my phones,never made any backups, and followed all the instructions properly.