GRUB consist of two parts (or stages).
The first stage resides on MBR, that doesn't do anything but load the second stage.
The second stage is on the root partition (or any other you decide too), this is what actually load your OS('s) and you can configure. If you already have GRUB installed on MBR, you should be able to only install it on the root partition, but if you are unsure, select MBR.
You're not drunk if you can lie on the floor without holding on.