There is nothing wrong with a v-neck on knit fabric, and facings are certainly the way to go. Staystitching the neckline is certainly a good option, but I use narrow strips of interfacing along the seamline to achieve the same thing (about 1cm wide). I actually have seam stabilising tape, which is strips of interfacing, with a row of stitching up the centre of the strip. Remember to interface the facing with knit interfacing if you have it. I actually block fuse interfacing to a piece of fabric first, and then cut out the facing, so that it doesn't stretch out when cutting. Also, remember to under-stitch the facing, so it doesn't roll out.