If you don't want to get banned, then do this:
-Turn off SpotPass Information Sending [Settings > Internet Settings > SpotPass > Sending of System Info]
-Disable "Show Friends what you're playing"
This stuff does not get you banned if you do this:
-Pirating games
-Using Homebrew CIAs
-Using VC injects
-Using any installed CIA
-Doing this even if you are connected to the internet
This stuff is stupid and gets you banned:
-Using online features of games that aren't out yet
-Cheating online
-Sharing your friend code on online message boards if you're hacked
-Sharing system specific information (LocalFriendCodeSeed for example)
EDIT: Check out
this thread also.