I ripped all my fabric for the clothes I made -- my clothing instructors insisted. I find I need to extra inches to "straignten" the fabrics that have been cut -- and then it is not a true straight of grain. If I have long pieces, I prefer to rip -- I know the edges are stable. If you want to know how wonky things are -- try straightening a panel piece. They are way off grain (and off any kind of square).
If the store only rips, then all of the pieces are on grain and it is a great feeling to know that you have a stable edge.