If you're smothering just grass, I'm pretty sure a foot of mulch would kill it. Now, if you're talking salmonberries or blackberries or bindweed, they'd grow right through. But, it should kill grass quite nicely. Make sure to mow the grass as short as you can, as the roots will die back.
As for needing a nitrogen source, it depends on what perennials you're planning on planting. If it's something like raspberries or blueberries, they probably won't mind too much. The nitrogen source would help break down the woodchips, and the grass underneath, and help ensure it dies. If you're putting something in that doesn't just adore carbon, then you'll probably want to add some nitrogen in there.
Another thing to realize is that the woodchips won't decompose that much over the winter...meaning that you'll still have almost a foot of woodchips between the air and the soil. Which means, when you go to plant, you'll either need to pull back that foot of mulch (perhaps unveiling some grass that had gone dormant and not quite died yet), or digging a hole in the woodchips, filling the hole with soil that matches the underlying soil, and planting in that soil. I did this when I planted my daughter's pear tree (I'd put a good foot of mulch over paper sacks to kill salmonberry), and the tree seems to be doing alright.
I agree with Nicole, no need for cardboard unless you have some woody type plants to get rid of also.
The woodchip - nitrogen depletion thing is more misinformation than truth, If you want to get those chips deteriorating make some mushroom slurries and pour those over the woodchips as you are spreading them.
I did a post about the woodchip myths a while back. If you are concerned, just add some spent coffee grounds to the woodchips, that will take care of any nitrogen needs you might think you have to address.
When I have done it with deep wood chips like you mentioned, when I am ready to plant, I open a hole in the wood chips down until I hit the soil below and fill it with compost, or a soil/compost mix, and then plant in that. I have had great results that way.