If you forget to do this, the server might get stuck at the Grub boot screen, forcing you to connect a keyboard and screen to get it going again.
Thankfully Grub 2 has made this a bit easier:
Log in to your server as root or use sudo,
Add the line:
Update the grub configuration:
The timeout value is in seconds, I use 10 seconds since that gives me ample time to enter recovery mode should it be necessary.