Also, there is a question of WHEN would Germany launch such an attack.
Prior to July 1940 is a no no. Can't pick ANOTHER neutral to attack, it would just bog down the attack on France to have to deploy more forces from Fall Gelb.
AFTER June 1941, all forces are needed against the Soviet Union. It would be madness to pull forces away from that front to defeat Switzerland (then again, when did anything Hitler did consider madness or not).
So between July 1940 and June 1941 perhaps?
This sends a clear signal to all neutrals that all bets are off. If you're not with the Nazis, you're against them. This may or may not be a help with the countries around the Balkan region.
((I know Hearts of Iron does it because of gameplay issues)) Who guaranteed Swiss independence? Would they (if not already at war) be prepared to declare war on Germany (yes, I'm looking at the USA)?