Accuracy is very important and will save you a lot of headaches when sewing blocks together. That being said, in a pinch, there are ways to "fix" a 1/4" mistake without starting over if it's worth the trouble. Sometimes, if you're not matching seams, you can ease in some of the fabric and make each side of the block a little "short" when putting your blocks together. If I do that, I will put a pin in the short sides so I will notice when I'm sewing the rows together and will know to line that edge up a little shorter with the edge on the other row. If it's a cutting problem (oops, cut too small!) you can take out one or two seams and resew them a little narrower than 1/4". I was helping a friend yesterday and this very thing happened. She was working from a kit and did not have a lot of extra fabric, so I ripped out her seams, resewed them and eased them into her block. It was a headache, but it worked and it was the only way to finish her table runner with the amount of fabric she had. So.... my best advice is to be extremely accurate in your cutting and sewing that 1/4" seam, but if something goes awry, if it's less work to "fudge it" rather than starting over or ripping out seams, I'd do that.