It's up to you if you wish to register a Nintendo Network ID on Nintendo 3DS. Registering a Nintendo Network ID has a whole host of benefits however: you can join the ever-growing gaming community on Miiverse, download free software and demos from Nintendo eShop, and combine your balance with Wii U if you register the same Nintendo Network ID on both systems.
A Nintendo Network ID is required to download free software and demos from Nintendo eShop on Nintendo 3DS, as well as to post content to Miiverse. If you wish to use these services, please register a Nintendo Network ID.
No, your friend list will not change if you register a Nintendo Network ID.
No, this is not possible. Your Nintendo Network ID is tied to your software and/or additional content you have purchased and downloaded. As the rights of usage for software you have purchased apply only to a single Nintendo 3DS family system, you can only register your Nintendo Network ID on one system.
No. Each Nintendo 3DS system can only have one registered Nintendo Network ID at a time.
Yes, this is possible. If you don't already have a Nintendo Network ID on Wii U, you can create one on Nintendo 3DS and use it on your Wii U at a later date.
To link an existing Nintendo Network ID to a Wii U user account, please see our Tutorial: How to register a Nintendo Network ID on Wii U.
After registering a Nintendo Network ID, you will be unable to change countries and use multiple currencies – for instance, pounds and euros – in Nintendo eShop. The funds in the currency that corresponds to the country setting of your Nintendo Network ID will remain, and any funds in other currencies will be deleted and no longer usable. Please make sure to use up funds in any other currencies before registering a Nintendo Network ID to your Nintendo 3DS system.