I'm with Darren, no you do not need to have a blank value. Especially if you change the default to some other value.
A perfect example of this is on the AP/AR Terms Code. Historically blank was used for 'net 30 days', but the problem was if you started with a non-blank, then if you wanted to go back to blank, the system thought you didn't put a value in and would put back in your old value. So to get around that short sightedness they also have a '30' for 'net 30 days'. Thus you end up with 2 different codes for 'net 30'.
The "trouble" with not allowing blanks is that it means extra keystrokes for your users. But if you set default values, and since this is a new install they won't know that they could have had blanks.
It will require more diligence and some upfront work. But it sounds like you've got the Governance committee to help make it work. If you've got the time and patience to test everything, I say good for you and go for it. Worst case scenario is that you have to go back and loosen the best practice in a few places. It's much easier to loosen than to try to reign it in later.