does changing

GRUB_DEFAULT=saved (in /etc/default/grub) and

sudo grub-set-default "Ubuntu, Linux 2.6.xx.xx-x-generic" (or whatever the
OS is you want to be default)

not do the job?

But really, the only reason I could think of to not edit it manually would
be to safegard a casual/new user against running update-grub for some reason
and overwriting all of their changes. I also seem to remember reading if
you remove an old kernel in Ubuntu with synapic that the update-grub command
is run automatically... Don't know if that is true or not... Has anyone
tried it?