Welcome to LQ!!
Most Linux installers give you the option of NOT installing a bootloader (or at least the option to put it on a floppy, USB stick, etc.)
Most Linux installers ALSO are quite good at detecting the other OSes present and configuring GRUB accordingly.
But doesn't that mean Grub gets installed on the MBR a second time, making a mess?
It doesn't "make a mess"---all that happens is that, when you install GRUB to the MBR, you overwrite whatever was there before.
I currently take the lazy way out--I let each installer take the MBR. Then, if it did not correctly set up for all my other installation, I just go in and fix it by hand.