I also started from Ubuntu 10.04.2 (checkbox 0.9.1), so I don't have checkbox-backend.
I verified that without -proposed, installing checkbox 0.9.1 does not install checkbox-gtk (nor -cli)
With -proposed, installing checkbox 0.9.2 also does not install checkbox-gtk nor checkbox-cli (neither with apt-get, nor with software-center).
However, apt-get does install 'Recommends' by default. I tested on aislerot, which is recommended by ubuntu-desktop:
sudo apt-get remove ubuntu-desktop aisleriot
sudo apt-get install ubuntu-desktop
apt-cache show ubuntu-desktop | grep -o 'Recommends: .* aisleriot,'
Perhaps the behavior of software center has changed in new releases, but this was a fresh install of 10.04.2