Here is what I've been taught, have talked about with other instructors and have read in regards to an intruder to your home while you are occupying it.
You're on your living room couch and someone breaks through the door, you're going to defend yourself right there right? Even in a retreat state, it's only while you can reasonably do so and you don't want to turn your back on an attacker, that wouldn't be reasonable especially if you have family in other parts of the house, you don't want to lead the attacker to them.
Now lets say you are upstairs and you hear someone down stairs (more than just a "bump in the night", but you know someone is down there by sight/sound). You can't hunt him down in the darkness and shoot him, that would make you the agressor and you'd likely be prosecuted. Plus you don't want to go shooting the drunk teenager from next door who just came home from his prom and went into the wrong house. Instead you take a defensive position, call 911, tell them you have an intruder in your house, you are afraid for your life and you have a gun, leave them on the line to record and issue the intruder commands to get out. As one instructor said "Get out of my house now. I have a gun, if you come any closer I will kill you!" usually does the trick. If they make entrance into your "safe" room then you defend yourself. Unless you took a defensive position at the top of the stairs or hallway, what would someone in a retreat state do differently other than making their stand in an actual room with no escape route?
On a side note... my "Self-Defense Laws of all 50 States" by Attorney Mitch Vilos just arrived yesterday and I haven't had a chance to read it yet, but it looks really good with is plain talk summaries, similar to the NY book I have that had commentary from a former DA and retired NYSP LT.