i always when adding corners to any block that will have (toss away triangles) stitch the original diagonal- then turn & stitch again on the outside along the diagonal- then cut between the seams so i have pre-made 1/2 square triangle blocks to use for borders, other blocks, other projects....so i cut both layers
sometimes people choose to only cut one layer-for added stability- it's your choice- but hand quilting it is more difficult to stitch through the extra layers.