I've seen two pages explaining how to backup & restore the MBR:
They've shown an example with restoring only the partition table (64 bytes):
dd if= sda-mbr.bin of=/dev/sdX bs=1 count=64 skip=446 seek=446
And an example with restoring the entire MBR (512 bytes):
dd if=backup-sda.mbr of=/dev/sda
What's the reason one wouldn't want to restore the boot code (the first 446 bytes)?