It isn't necessary to mount the boot-partition under /boot, it just makes life a little it easier. Then when you want to edit the menu.lst, or copy some new kernel you always have your boot-partition accessible in the same place.
It doesn't mess up anything, as long as you only mount the boot-partition.
When you install a new distro, don't install grub in the boot-partition, install it under the new systems /
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