I agree with Gale you can pay and get badly cut stripes. From the outside of the roll you can't tell. If you are thinking they are cheaper think again I find some stripes I just can't use because I don't like the colour or pattern.
For a newbie cutting your own could be expensive as you have limited stash so I would suggest you keep buying for one pattern at a time but buy an extra half or yard of the fabric to build a stash.