Every time the series started I remember wondering about the lock on Dexter's door. I was sure I had the same one in my garage so I went to check and it turned out to be a "Dexter" lock. Is there a specific meaning to this?
Well, it did look pretty solid...
But no, I don't think there was a special meaning - apart from making Dexter's normal morning-ritual look "gruesome" by using weird camera-angles.
Also, remember that the whole opening credits - plus a bit extra - were shown in one of the episodes of the first season, to show that "life is good, and everything is great in Dexter's life"... It was also parodied in a episode right after Dexter's son is born, and he's extremely sleep-deprived due to the baby - his t-shirt got baby-food/burp stains, his shoe-laces snaps, and he looks dead tired.
I've always taken the opening credits of Dexter as a way of the producers to show that, on the surface at least, Dexter is a normal person. He sleeps, wakes up, has breakfast, gets dressed and goes out. Just like you or I would.
The showing of locking the door I'd always taken to mean that even when someone is a serial killer they don't take how dangerous they are for granted and assume people will know it's the home of a serial killer, and therefore they shouldn't break in.
Dexter is a normal guy with a normal routine. The lock will be just a normal, everyday lock.
Why do they show this? Because it's showing that Dexter is very careful to blend in with everyone else. Don't stand out and don't give people a reason to remember you.