Into the Deep!
This is me 12 years ago looking at a new hive. See how white my suit was?
The smoke drove the bees down but when we opened the hive the top of the frames and the inner cover were full of bees. See all the wax on the tops of the frames? That's a sign that they need more space to build wax.
If you started a brand new hive your nuc should have expanded in the first deep building out the bare foundation on at least 8 of 10 frames. The frames should be full of brood in all stages. Now it is time to add the second deep. And keep feeding. Remember you are investing in their home. They'll 'draw from the bank' if they need it to build more comb.
Why don't you just add two at first? Bees tend to work upwards. If you add a second deep too soon, without the first being built, they may chimney. This means the bees will work up, not out, and the outside frames won't get built or be poorly built. Those frames won't be able to hold the honey and pollen stores which need to be gathered in the late summer and early fall. That could spell disaster for the hive.
There's other beekeepers who had drawn comb. That is like gold. The hive grows like crazy because they can put all their resources into making bees. You probably have had the second deep on for a while. If you have 4 to 5 frames of brood plus honey and pollen in your second deep it is time to add a honey super.
Should you use a queen excluder?
I talked with a beekeeper in just this situation this afternoon. She came to purchase another super. "The bees did not build the super out last year but they are this year. We didn't put the queen excluder on."
The problem here was she put the queen excluder between the deep and a new box of foundation. It kept the queen out. But it also kept the young wax building bees from bringing their wax through. An excluder is a tight fix for the bees. I believe the wax the young bee excretes gets rubbed off by the excluder and so no comb can be built. Only use a queen excluder between a deep and built out honey supers.
We use queen excluders. It keeps the queen out which means there is no brood where you want honey. Brood in the super when it is time to extract is a mess. It also reassuring to know that the queen is not up in the honey super when you want to take it off. Who wants to look for the queen or take the chance of killing or damaging her when pulling honey supers? Some will call them honey excluders but they have never hurt our honey production. Some of our hives have filled two and are working on the third super already.