Not sure why I singled out grub1 to not add that argument, it is
documented to work and does. By not setting it, old CentOS images
that used grub1 would hang at boot time for > 5 minutes causing a
stated timeout and reboot. Once stated had tried that 3 times, it
would stop doing the reboot and eventually the node would come up.

I have no idea why it was hanging unless the console output was
going to the real serial port (com1) and that was either generating
garbage input causing grub to go interactive or causing output to
block.