What the book is talking about is getting the fabric straight on the grain so that any cuts you make will be straight with the proper balance. If the grain is not straight the fabric can bunch up from the bias side or if you are cutting long strips and the fabric is folded in 1/2 you may find an uneven hump at the fold when you open you cut pieces. It is an old habit from dressmaking days to straighten the fabric. I still do it with my quilting fabrics. Especially when I prewash my fabrics. Now I don't cut off 2" since I am hopeful that my grain is not that far off. But it could be off by 2" on one side and only 1/4" to the other side which is why you straighten it. I will line the salvages edges first so I have a visual straightening before I lay it down make a notch and then tear it across. The more difficult part is making sure you keep the fold straight and press it flat. Pinning helps.
Now do I do this all the time no. I do how ever because I am a bit OCD do it more than 50% of the time. Ligning it up on the grain makes for a much nicer block when squaring up your pieces and your blocks.