After hearing some stories about rear diff problems when my V was newer, I requested my dealer change my rear fluid early... He said it wasn't needed until the MM indicated and suggested I wait. Unless you are experiencing some kind of problem or unusual noises, I'd suggest you wait - 18K is a bit early for a new vehicle. I also would be suspicious of that dealer, seems a bit money hungry.
That said, many miles later I did start to hear a groan from the back and changed the fluid which resolved the problem. Now that my V has almost 130K I have taken to changing it every other oil change (~15K miles). Same with ATF. I'm sure that is overkill, but the fluid is cheap and 30 minutes of my time is free so what the heck if it makes me sleep better at night.