Let's discuss facings first. You know what a facing is right? Well if you don't have a look at your pattern inventory for Simplicity 1880 and pick out pieces #3 and #11. Those are the facings for the wrap dress and they are inserted along the neckline. I've got nothing against facings when they are well used in a pattern, but I find all too often that facings are very much overused. This is only my opinion. I have a wrap dress that I made a few years back and it has facings and the facings, I do not love. Facings can be fiddly and they can pop out and be lame and weird. So I say, forget the facings.
Use self made bias tape instead. This works especially nice if you're using a drapey type fabric, which is what the fabric suggestions call for anyway. You can forget interfacing for the wrap dress too because if you use bias tape in place of the facings, you've then forgone the need for interfacing throughout the entire dress and that is something to write home about. Self made bias tape is really easy to make and you can follow one of these lovely tutorials put out by Colette:
How to Make Bias Tape
Continuous Bias Tape Tutorial
You'll probably need a good 2 yards of bias tape, I would say and that really doesn't take very much fabric. You could also put in a contrasting bias tape made from another fabric instead. I'll be making and using 1/2" wide single fold bias tape. But you can also do it Tasia's way and do what she did for her very first pattern, Pendrell which uses a bias binding:
Pendrell Sewalong: Attaching Bias Binding to the Neckline
Now, you are welcome to use the facings for your version of the wrap dress too and I won't think the worst of you in the least for it. If you plan to do the bias tape with me, I'll give proper instructions for the construction aspect of this along the neckline.
Please take note that this same idea does not apply to the shirt dress. That's a very different matter and one in which the facing - which is attached to the shirt dress front - will be used and is necessary. Make sense?
Hurrah for Bias Tape!