The first stitch in cast-on rows on flat knitting always look rounded. I saw a tip somewhere for making the corners more square, which makes a nicer seam, but I’ve lost it.
Just came upon the answer at Ravelry:
“When I do a cable or knit cast on, after pulling through the loop of yarn that will be my last stitch (while it’s still on the right needle), I bring the yarn to the front between the stitches, then slip that last stitch onto the left needle.
It makes a nice ‘square’ corner.”