Be mindful of where you create an ID.
http://www.nintendoworldreport.com/news/32536
When Nintendo outlined its Wii U account system earlier this month, we learned you can create up to 12 unique IDs per system.
With the launch of the console today, though, multiple users report that once a Nintendo Network ID is activated, it is tied to that specific system, and can't be used on another. Attempting to do so brings up a warning that the respective ID "has already been linked to another Wii U console."
Nintendo's official Wii U support page confirms the limitation. The page also carries instructions on how to deactivate a Nintendo Network account, which removes the account as well as associated information, including eShop activity and balance, Friend List, Miiverse activity, and registered email address. Any username tied to a deactivated Nintendo Network account also can't be used for a new Nintendo Network account.
Once Nintendo expands Nintendo Network and Miiverse access to other devices (3DS, PC, smartphone, tablet), this wont be a problem anymore.