Congrats!
I use Ds frequently at highway speeds, usually when I want to pass. Gives the added power to really punch it quickly, and is more responsive to the gas pedal.
Think of it doing 2 different things:
1) Jumping up the RPMs, effectively doing the same thing as "down-shifting" a gear in a Manual Tranny.
2) Shortens the response time of the throttle, meaning that when you step on the gas, it reacts quicker than in normal D mode.
I consider Ds to be a "sport" mode, where you're essentially telling your car that you want some power and better response.
I don't see a problem using Ds at any speed, as the RPMs are still well under redline. I wouldn't use it ALL the time, because then you're engine is running hot and you're gas mileage will go way down.
The LOW gear (L) is a simlar change, but much more so. You're going to have very high RPMs in that gear no matter what, and the resulting engine noise is pretty irritating.
The only times that I have used L is when pulling my utility trailer slowly over dirt/grass/gravel when I need the added power but am not cruising (I also lock the AWD on in these cases).
As for "engine braking" the Murano has something called "Downhill Assist" which automatically does this in D. What it does is try to maintain your speed. So if you're going downhill and are picking up speed without pressing the accellerator, it will jump the RPMs up a bit (downshift) to hold you back. It doesn't use a LOT of force, and you'll still pick up speed (especially on a steep hill) but you'll feel it kick in periodically. You CAN use Ds to increase engine breaking, but I prefer using the actual brakes. The Mo's breaks are very good, and as someone else mentioned elsewhere on the forum, a brake system on a car is one of the most powerful systems, and it's completely designed around slowing and stopping the car, so why not use it for that?

Brake pads and rotors are relatively cheap (compared to the transmission) so might as well use them.