I press seams open when constructing a kaleidoscope block that has a circle inside a square. You can't nest those seams. Then stitch in the ditch which was the only technique I knew. That was BQB (Before Quilt Board)! Bed sheets don't hold up to a lifetime of machine washings. When the borders on a 20+ year old quilt (made of bed sheets) literally disintegrated, I just cut them off. I don't use sheets. I added a pic of newest kaleidoscope made with open seams.