Yep, forgot about that stupid "let met check if you can format" thing on the 3DS. Don't take the risk and go with @Ricken's advice!
Great, thanks. The page here (http://www.3dbrew.org/wiki/System_Settings#System_Format) says that:
When you first enter the System Format menu, it will check if a NNID is linked. If there's a linked-NNID, it will then display: "Are you ready to connect to the Internet to check whether data can be formatted"? Continuing will only result in connecting to wifi for checking in with Nintendo's servers, which may fail if the console is banned. Once that's done it will continue with the usual system-format messages; proceeding will result in the NNID cookie, potentially still present on NAND backups or multiboot scenarios, being invalidated until the next sign-in (at which point even old sessions will be valid again)."
But regardless, I think I'll use TinyFormat just to be safe.