Any time you are making ANY blocks, you need to be sure that ALL of your blocks measure the same as all the others. Since everyone sews differently, and even different fabrics can behave differently, the blocks may differ in size slightly.
(We do D9P blocks for Quilts of Valor, send kits home with people. When they return the blocks, there is a WIDE discrepancy in sizes (sometimes up to 1/2" or so). If we tried to sew those blocks together without squaring them up, we would get a very uneven quilt top.)
Also, the edges of the block are often not straight and true. By squaring up BEFORE you cut them, when you turn the two pieces you will have nice straight edges to match up with the freshly cut pieces.
To square up your blocks, measure them to see what size is most common. I like to use a square ruler that is larger than this size, and put the halfway mark of the ruler on the center of the block (for an 8" block I would line up the 4" mark in the center, both L to R and top to bottom.) Trim the right and top edges. Then turn the block 180 degrees. Now line up the 8" mark on the left and bottom sides and again trim the right and top edges. (These directions are if you are right handed.)
Note that you are trimming a little bit from all four sides, rather than a bigger portion from just two sides. If you just trimmed from two sides, your block would end up uneven instead of symmetrical.