if you don't have sleep mode or wifi on (Duh, who would)
actually sleep mode by itself isn't that back, but have wifi on with it at the same time, then yes, it's bad.
but if you disable "spot pass auto download" then you won't get nags and such,
plus having the wifi on for only the time limited to accessing gw menu for such stuff as hand backup or such and booting into emunand, and then turn it off you'll be update free
and N won't auto update your 3ds/2ds without asking the end user first,
it'll only update if "you" accept the update message Pressing A "yes",
if not it won't forcefully update without you letting it to.