With many block patterns, keeping seams consistent is enough (Mary Ellen Hopkins made that point with her books) as long as you understand how to adjust your cutting. However, when you get into complex blocks and especially blocks with certain arrangements of triangles, mastery of the scant 1/4-inch becomes important.
You need to understand that the desired finished result is a "perfect" 1/4-inch seam. That is how all of the pieced block calculations are made. The problem is, if you sew a perfect 1/4-inch seam, the end result will be too small because your sewing thread requires a little bit of that 1/4-inch and turn-of-the-cloth when you press the seam requires a little of that 1/4-inch. People use different weights of thread in their machine, and there are even bigger differences in how people press their seams, so one person's "scant" 1/4-inch will be different from another's. I would say 2 threads less than a perfect 1/4-inch would be the absolute minimum; most people need to be a little more scant than that because of their pressing.
There may be a few people who can eyeball a scant 1/4-inch seam consistently, but I am not one of them. What I do is create a physical barrier made out of moleskin. Check other of my posts to find a detailled description of how I do this and test that it results in a "perfect" 1/4-inch seam.
Incidentally, I was just at JoAnn's yesterday and found a Dritz seam gauge on the notions wall in the quilting area. It is a little red ruler with indentations for your needle in order to set up a physical guide for different seam allowances. I measured their 1/4-inch indentation on the ruler, and it was an "exact" 1/4-inch. This means that this ruler gauge is useless for quilters, as all of your seams will end up too big (and your pieced block too small)! I did purchase their foam guides (you use the ruler to get an edge, then glue down the foam guide as your physical barrier), as they appear to be taller and harder than my moleskin. These adhesive guides should actually work better for me than moleskin, but I haven't tried them out yet.
If anyone wants to see my moleskin guide directions and can't find them, I will look later and post a link to the old post.