I try for a perfect 1/4" seam allowance all the time simply because it prevents problems down the line. It is worth taking the time to perfect that seam allowance -- saves so much aggravation!
Having experimented with many methods, the one that *always* works for me is to create a physical guide for the seam. I used to cut adhesive moleskin (available in foot section of pharmacies), but now my preferred physical guide is this one:
http://www.amazon.com/Dritz-Quilting...dp/B0085L2Y2I/ . With this in place my eyes are given relief (because they do not need to be glued to a mark on the bed of the machine) and I can sew very fast and still get perfectly even seam allowances.
Also, I have found that using fine thread helps a lot with my seam allowances. My favored thread is Aurifil 50wt 2-ply on top and in the bobbin (or 60wt bobbin thread in the bobbin). I think this helps because the turn-of-the-cloth is able to be ironed with more precision.
Recently I started piecing a kit and forgot to move my needle over one notch for the seam (necessary on my Bernina because of the relationship between the needle and the feed dogs). I did not realize it for a long time, when I went to match up seams and they didn't match. This was because every seam allowance was double a single needle-notch larger than they should have been. I should have ripped all my stitching out and started over! Instead, I decided to "adjust" the pattern as I went. It has been doubling my time at the machine; wish I had started over with the correct seam allowance. The pattern has lots of cornerstones, so it is tedious to adjust a seam every few inches!
In general, if blocks can fit together without ripples, I don't worry too much about exact size. I would guess I can adjust up to 1/4" difference in most 12-inch blocks without a problem. With 6-inch blocks, the tolerance would be closer to 1/8".
Edit: My experience with this quilt kit is proof that having a consistent seam allowance is not enough. It works for specific patterns, but definitely not for patterns that combine blocks with sashing and cornerstones!!!