you can't boot to raid5 at all AFAIK. When you have a RAID1, grub is generally just accessed as a single device in teh first place. Whilst you have this /dev/md0 block device, the filesystem on each device is still complete and can be read as whatever filesystem it is when mounted. With Raid5, there is no complete filessytem on any device.
/boot makes no sense to raid anyway, just keep it separate and it'll be fine.