It is as important to have an exact 1/4" seam as it is to cut your fabric accurately.
It may seem like a waste of time to check your machine and make sure that you can sew an accurate 1/4" seam (or scant 1/4" seam depending on your thread, fabric, and sewing machine) but it is well worth the time it takes to have your block turn out as close to the finished measurement as possible. Not only will your block look nicer when it is finished, it will be the right size and your quilt will go together much more easily.
Here are a few videos showing how to get a perfect 1/4" seam.
Here's a video explaining chain sewing blocks as well as pressing (versus ironing) blocks:
