What you are thinking of is that the interior has no foliage so if you cut back beyond the outer layer you end up with bare stems that may not sprout vigorously enough to fill back in again well or even remain bare. How far down you can go depend on how far down your plants are green.

Isn't the question really about "topping"? My understanding is that you can (there are sheared arborvitae hedges, after all) if you're prepared for the tree to have a flat top and to eventually be apt to grow multiple leaders. It's not too likely to kill the tree, and topping won't make for bushier bottom growth. And you can use a pruner to taper the remaining top greenery to lessen the effect of the topping.

There are quite a few university extension services with on-line pruning guides. Most of them will prefer selective pruning to topping arborvitae, but none say that topping is NEVER the answer.

I did 'topped' arborvitae 3 or 4 years ago and see nothing wrong with a plant. At once it was looking somewhat funny with flat top, but in a year or so it developed somewhat oval top and I have no problem with such appearance vs conical-pointy one.
Mine was over 10' tall and it was reduced to 7' at the time of topping. Now approaching 9'.

thanks, I was referring to the flat top but I forgot about the dead leaves also. BUT that said I am going to do it. Thanks again. It's great to have all this experience at your fingertips. :)