There are two parts to this process. The prep part and the assembly part. The prep part is when you use rough cut fabric pieces and sew them together into a joined fabric section ready to be cut out into a "Block". Then you have the assembly section which is where you sew the "Blocks" into rows and the rows into a quilt block.
In the rough cut process the seams are not important except for being straight seams. It doesn't matter how wide or narrow your seams are when you sew the rough cut triangles together. You are not matching seams to anything in this process. So they just have to be straight. No stress no mess. I sometimes trim them after I have sewn a seam so there isn't too much bulk. But I don't always.
Now in the assembly process - this is when you are joining two Blocks together - you do need to watch your seam allowance. This is where the accuracy of your seams does matter. It still doesn't matter what seam allowance you use as long as you use the same all the way through your process. If you need to end with a perfect 12 1/2" quilt block you might want to be careful then to keep to the 1/4" seam allowance rule. But if you are doing it for a project of your own or you are making a quilt just for you if the quilt block comes out 12 1/4" instead of 12 1/2" then just make them all that same size. It doesn't have to be stressful. Sometimes you can just say oh well and move on.
Here I have 2 rough cut triangles. This means I didn't bother to take time to cut them out perfectly.
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I laid them together face to face and I sewed along the one edge (in this case the blue one) and used the blue edge as a guide ignoring the white edge. Now I didn't use any particular seam allowance just made sure it was a straight seam.
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Here you can see my seam isn't perfectly 1/4" from the edge of the blue. But it doesn't matter. It only matters that the seam is straight and not crooked.
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You can see the white side is wider at one end and narrower at the other. As we are going to cut out the square from this it doesn't matter what the seam is like.
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Now this is only half of the Boston Block process and you will need to follow the rest of the Boston directions by trimming and adding the bottom triangle. But here you can see when the template is laid on the fabric and you cut out the square from the center - the seam behind the joined fabric doesn't affect the outcome of the Boston Block at all. So you don't need a perfect seam allowance when putting the rough cut triangles together into the joined swatches ready to be cut out. If the amt of fabric in the rough cut seam bothers you - then you can trim it down if you like. I think the rough cut process elilmates the need for perfect measuring and saves time and energy when you don't have to fiddle to make it perfect.
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