Have you ever heard the term fussing cutting? It's kind of a funny term. But if you are new to quilting it might be new to you.
Fussing cutting is when you cut a specific part of fabric out of a larger piece to highlight or show off a certain design. 
Usually when cutting fabric we just cut by measurement and start with the next part of our yardage. However, with fussy cutting, you decide which flower or animal you want to be the center of you block and cut that specific part out of the fabric.
The upside of fussy cutting? Your quilt turns out beautiful with blocks just how you want them.
The downside? It takes a lot more time and it also wastes fabric. After you are finished fussy cutting you are basically left with fabric with a bunch of holes in it.
When should you fussy cut? Any time you want to! It's your fabric and if you want your blocks a specific way then I say go for it.
Tips
When fussy cutting DO NOT forget each cut needs a 1/4 inch buffer for the seam allowance. It's important to cut your blocks with this in mind. I like to tape my ruler with the size I am cutting just so I can remember. Then I make sure the design is 1/4 inch inside the taped area all the way around.
There are also fussy cutting rulers available to help you get each cut just right. You can find a set of square fussing cutting rulers here. 
Before I have made half square triangles extra large (as above) so I can cut them down with the design I want. In this case I was trying to get as many strawberries as I could on each half square triangle. Again, this is a waste of fabric, but in the end I really love all the strawberries in my quilt.
If you want to try your hand at fussy cutting and quilting with directional fabric, Auntie Anita's Patchwork Quilt is a great beginner pattern to learn with. The pattern is scrap and fat quarter friendly. The above blocks were cut for this pattern. You can get a copy of the pattern here.
Most of all don't be afraid to try something new and to mess up.
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