In the wp-config-sample.php here is the line for DB_Host:
define('DB_HOST', 'localhost'); // 99% chance you won't need to change this value`
If you are hosting on Godaddy.com or 1and1.com you will need to change the DB_HOST. I don't know the exact percentages, but I'd imagine that those two hosts make up at least 25% of the websites hosted. (Not to mention the many other hosts that I haven't checked)
Can you change 99% to a different number?
I realize this is ridiculously minor, but every time someone else has installed WP for a client, they don't change that value because 99% is pretty convincing.
Thank you for your time and all the great work you do,