You want to trim off only what's needed to result in an 8" square. You're right, it won't necessarily be the same amount on every block.
If you have a square up ruler, that would be easiest. Mark it with tape at the 8" markings. Then lay over the block with the center point in the middle of the square. Use the diagonals on the ruler to point toward the corners. Before you start trimming, make sure that you are happy with what is showing in the marked off 8" square. You might also find some reference points and put tape on the points so it will be easier to use the ruler on the next square.
Then trim around two edges of two sides of the ruler. When done with the first two sides, turn your ruler so that the taped edges are directly over the trimmed edges. Then trim the other two sides.
I know I'm not explaining this well.
http://www.youtube.com/watch?v=cdUTLkNc0po
http://www.youtube.com/watch?v=YuqauFfA5xk
Try watching these. Or enter Squaring Up Your Blocks in youtube.
Good luck.