I used to partition drives for TM backups, but I found when the backup drive is sized more than double the combined drive sizes of machines being backed up, partitioning adds nothing except some complexity and constraints.
There are other ways to limit the size of the sparse bundle, forget the best methods off hand.
like: sudo hdiutil resize -size 250g
or as discussed here:
http://pondini.org/TM/A8.html
Some when faced with mixed TM backups and user data used Disk Utility to create Disk images sized for the desired TM backup max and then mounted the images and pointed TH to it. Used to be useful if you wanted to encrypt the backups, as DiskUtility provides the capability to encrypt disk images.
Partioning will work, just saying that there are other methods available that don't lock the drive down so much. When you change your mind about what's the right partition size allocations, or add another computer, its reformat the entire drive time. Which may or may not be a big deal for you.
