I do think ripping gives you a more straight of grain edge - but ripping selvedge to selvedge will likely leave a half inch or so unusable. So my instinct is to say that they need to allow for that, and give an inch or two extra. In thinking about it, though, I often have to cut that much off to straighten the edge anyhow, and getting it to the point of knowing where the straight edge in can be challenging, esp. for longer cuts.
This reminds me of waaayyy back when I started sewing - in 4-H. We were doing garment construction and after buying fabric at the Ben Franklin, we had to pull a thread up with a pin and then pull it from edge to edge to get the straight of grain for our pattern. :shock: