From my experience you should install the bootloader in the hard drive itself instead of a partition, most likely /dev/sda (unless you are using a mac with OSX where you need to install EFI in the hard drive and create a partition for the bootloader).
Every time you add a new distro grub checks for installed OSs in each partition and it will update your grub screen with an option for each OS.
The only thing that will happen is that some distros have a different look and feel for grub so it might not be as nice depending on what partition you install last.
Finally if you want to update your grub (because you broke it or don't like the looks of it) you can put a live cd (or usb or whatever you fancy) and fix it. There are some guides for it. My point there is that you can play around without actually losing information though backups are always in place ;)
