With foundation piecing, I do not worry about biase on the edges of the blocks, the foundation under the pieced fabric stabilizes the block, so I just cut fabric to fit the space. If you are using the foundation piecing on paper instead of on fabric, where the paper is removed, I would worry only if I was doing a quilt without a border. I leave papers on until the quilt top is together and the straight grained border would stablize the block after the paper was removed. With no border to stabelize the outer edge, I might want to make sure the outer edges of the outter blocks were not biase if they were large pieces. For those sections of the block, I would trace a paper pattern of the shape, add the 1/4 inch seam allowance and mark the straight of grain for the edge, then cut and sew in place. A lot of work, but if the biase edges have been a problem in your quilt, this should help.
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