Fabric is fluid. If your blocks are large, you can often stretch
two blocks that aren't quite the same to fit. However, this results in one
side basically being "gathered", and it will result in unsightly puckers, even
if they are small.
The goal in piecing is to make your work as flat as possible.
When you oversize a piece, and cut it down, your block will be perfect. You
don't waste much fabric, but it is an extra step, and not all quilters do it.
It you are cutting down a block like a half square triangle, you
can use a square ruler and make just one cut. However, if you are cutting
down a block like a 4-patch, which has a line down the middle, you must remember
to divide the amount you are going to cut down by two and cut off the same
amount on each side of the center line.
The goal in piecing is to become so good at cutting fabric, and
sewing with the 1/4 inch seam that cutting down is not necessary. Until
then, "cutting down" is a way to make a very attractive quilt that goes together
Here is a picture that illustrates cutting down a half square
triangle. You would not oversize a block this much, but I wanted to clearly
illustrate the lines. Notice how the diagonal line is matched to the fabric, was
well as the 3 inch horizontal and vertical lines. All you need to do is
make one vertical cut on the ruler and one horizontal cut.