Just realize that the marks on your cutting board may be different than a ruler. The moral here is: whatever you are choosing to use to measure the block, make sure you use the same method for ALL of your blocks. They may end up the perfect size, or just close to the perfect size. However, if your blocks are all the SAME size, it doesn't matter. Your project will come out fine.
Keep this in mind as you choose to buy rulers. Try to get them all from the same line; for example, virtually all of my rulers are from Omnigrid. When you do a project, make sure you use the same ruler for all of the cutting. That's because rulers can be slightly different, and those slight differences can result in unequal block sizes.
Also make sure you know how to sew a 1/4 inch seam. If you use the same size seam throughout your project, it will help immensely with your accuracy.
Good luck to you, and happy Quilting!