I love those signs, however I would post it ONLY inside my house or in the backyard (if it is closed off and hard to see).
If they were to break in and then see a "This home is protected by the 2nd Amendment" sign, they might have a change of heart. Or freak out and allow you to take the upper hand.
Edit: Long story short, IMO don't blatantly advertise outside, but being playful or tactful indoors won't hurt.
I told the new me,
"Meet me at the bus station and hold a sign that reads: 'Today is the first day of the rest of your life.'"
But the old me met me with a sign that read: "Welcome back."
Who you are is not a function of where you are. -Off Minor