If you read the reference :
http://www.justanswer.com/bmw/4qd9b-...2007-335i.html
It is talking about re-coding not repeat not registering your battery.
You do need to register your battery if you want the BMW Inteliigent Battey Sensor (IBS) to provide the current algorthim for a new battery not one that is currently in there.
The choice is up to you but you can register yourself as I referenced the site above. I have done it and it is easy to do.
What happens if you don't? No idea but most likely will not last as long as. The saving in not registering the battery might allow you to pay for a new battery say every 3 years.
No matter what you read Bosch makes batteries for BMW under the Excide name. Douglas is the other supplier. You will not find anyone but a dealer selling the OEM batteries and that dealer must be local since I don't know of one dealer that will ship to include getbmwparts.com.
You have a $50,000 car so what battery you use is up to you.
If you change your own battery be careful so you don't short out the IBS. Also there is a good likelihood of blowing a fuse located behind the glove box so if you don't have instruments when you restart your car check the fuse box there.
As long as you are careful you should be fine...