I tried installing Ubuntu 9.10 to /dev/sda a long time ago. For some reason Linux was the only option listed in the Grub boot menu and when I selected it my computer froze. My computer was no longer able to boot Windows. I could be wrong, but on an EFI system I think the MBR is just there for backwards compatibility. It's sole purpose is to point the computer to the EFI system partition where the boot files are stored.