I am not MTU, I am just a happy user of its products.
There is no reason to upgrade to DirectX 9.0 (although I did) if you have DirectX 8.1 and your system is working fine.
On the other hand, there is no reason not to, if your graphics card drivers supports DirectX 9.0
Either way, you will not see any improvements with one over the other. If you want to upgrade, that's entirely up to you. It is not needed to run MTU products, at least not yet.
Good luck,
Jon