It is advised to install GRUB bootloader to MBR (this will replace current bootloader from MBR) after installation. Because MBR tells GRUB to load when the system is booted.
I found this how to make it react for MBR
# This is fairly safe to set, it makes grub install automatically to the MBR
# if no other operating system is detected on the machine.
d-i grub-installer/only_debian boolean true
# This one makes grub-installer install to the MBR if it also finds some other
# OS, which is less safe as it might not be able to boot that other OS.
d-i grub-installer/with_other_os boolean true
and this if you don't want to install in MBR
# Alternatively, if you want to install to a location other than the mbr,
# uncomment and edit these lines:
#d-i grub-installer/only_debian boolean false
#d-i grub-installer/with_other_os boolean false
#d-i grub-installer/bootdev string (hd0,0)
# To install grub to multiple disks:
#d-i grub-installer/bootdev string (hd0,0) (hd1,0) (hd2,0)
these configurations are taken from Debian Lenny's preseed file here
this one is helpful for other examples