Front screen door is always locked when we are home. More for the fact my son likes to go for a walk up the street. Back door is not, but we have really high fences so it would be hard for people to access the back.
If anybody tries to come in while we are home, they'd probably be scared off by the dog as she is always inside when we are home. So many door knockers get scared by her as she's a staffy, but she would just lick them.