Is Grub installed to all the MBR of a RAID array by default by the Debian Installer?

Not sure if the installer does it by default, but if (once the system is booted) you run:

# dpkg-reconfigure -plow grub-pc 

just hit enter until you're prompted for "GRUB install devices", then you can select the MBRs of both drives.

Here is a screenshot of the relevant screen in Debian Jessie (8.3), with GRUB2 version 2.02~beta2-22+deb8u1. This shows the setup of my computer, which has two drives (/dev/sda and /dev/sdb) in a RAID 1 configuration (/dev/md1), and two other drives (/dev/sdc and /dev/sdd) in a RAID 1 configuration (/dev/md0).

If you install Grub legacy, then it's only installed on the one boot disk. You can add it to the second disk's MBR by running grub-install /dev/sdb (assuming that your second disk is /dev/sdb). Make sure that in both cases, Grub reads from the first BIOS disk (you may need to tweak /boot/grub/device.map for that). If one of the two disks fails, whichever disk is left will be the first BIOS disk.

You can install Grub2 on Debian squeeze, and I recommend that if you have any sort of nontrivial setup such as RAID. Grub2 has the advantage that you can put your whole system on LVM, you don't need a non-LVM /boot partition. As far as I know, the squeeze installer supports installations over RAID+LVM with grub2 (you may need to coerce it a little not to create a separate /boot partition).