If you have a pattern you want to make, then just select the fabric you want for that pattern, no matter what line it is from, or what shop you buy it at. Sometimes you may need to go to more than one store to get just the right fabric, because they don't always carry every line or every color.
When I first started, I would buy extra fabric depending on how much I needed for my pattern. I didn't always have time to work on them right away, and by the time I got around to them, if I needed more fabric, they would be sold out, and I couldn't find it. So if I needed a couple of yards, I would add up to 1/2yd extra. If I only needed 1/2 yd or less, then I would add 1/4 yd. If I really liked one of the fabrics I choose, and thought I would want it for another quilt, then I might buy a couple extra yards.
When you are just starting out, and you don't know what types of quilts you like, it's hard to build a stash. I have changed my mind on what types of quilts I like many times, and the kind and amount of fabric I would buy changes with the types of quilt.
As for cutting like others have said, you want to cut on the straight of grain, not the bias.