Numbering scheme:

1XXX: Preparation hooks (e.g. create-locales-files)
5XXX: Update/regenerate cache files (e.g. update-apt-file-cache)
8XXX: Remove files that should not be in a live image (e.g. remove-dbus-machine-id)
9XXX: Remove files that reduce the size of the image (e.g. remove-python-pyc)

This leaves three ranges 0XXX, 2XXX-4XXX and 6XXX-7XXX for your own hooks (but you can still use all the other numbers as well)