I've never given it a thought
Well, next time you can't sleep....
As it is rolled onto the bolt, the fabric will move about a bit. If you unfold the piece you bought you'll probably find the cut edge isn't exactly at right angles to the selvedge. Unless you realign the fabric - easier to do if you've washed it - then any wof cut will be off grain. This isn't necessarily a total disaster, but getting the grain straight reduces fraying and stretching.
Cutting parallel to the selvedge will give you on-grain strips and less chance of stretching.