Discharging static to the case works to an extent but the surface charge can reappear at harmful levels immediately. Those harmful levels won't even display a spark when you touch the case but they are enough to take out a board.
Unwrap a pack of crackers or a music CD and notice the thin wrapper clings to your hand but when you touch ground it doesn't spark?
That's enough surface charge to fry your board. Get a wrist strap, make something yourself or at least work barefoot anytime you're dealing with boards. Just make damn sure there is no power active anywhere.
Of course the rules for working around HV are pretty much the exact opposite
