My boy will bark at a sound or knocking at the door, etc however there's NO WAY (yes I say this with known certainty) that if someone broke in he would do anything, he'd run and hide absolutely for sure, however he does bark.
My female on the other hand, she barked when we first got her not sure what happend but she hasn't barked in YEARS now at anything and my husband says she's an overgrown cat...unfortunately he's right, she's old, slow, docile, and doesn't bark so she's in the negative department for guarding.
Now our first Boxer, he was a rescue, around 2 or so when we got him, was absolutely a guard dog and would have ripped someone's arm off before he let them in this house if they weren't supposed to be here (yes again know this for a fact) he was a fantastic guard dog but a great family dog too, very well rounded.