A little late, but I just noticed in the package manager GUI you can select unclutter and in the menu go to Package > Configure... and it gives you the option to set whether unclutter starts automatically.
You can also right click on it and hit Properties and there is an Installed Files tab that will show you its files (from which you can probably guess how it starts up).
No need to use Perl like that other thread suggests... you can just edit the config file:
# /etc/default/unclutter - configuration file for unclutter
# Set this option to 'true' if you want to start unclutter
# automagically after X has been started for a user.
# Otherwise, set it to 'false'.
# Options passed to unclutter, see 'man unclutter' for details.
EXTRA_OPTS="-idle 1 -root"