Believe it or not, I actually sat down one day and did a test like this to see how characters would sort in memos. It looks like categories sort them the same way.
The bottom three characters on my Palm keyboard are the null sign (3rd to bottom), Beta symbol (2nd to bottom), and trademark symbol (TM) (absolute bottom.) (Sorry - not sure how to make these on a standard keyboard.)
If you need or want to stick with what's available on Palm graffiti keyboard, numbers will sort the lowest - as Pam suggested, each number in succession will drop it that much lower.