i've never had trouble with corners slipping on the blankets i've made as far as i know. no one's complained or mentioned it happening. but still, i've begun taking the extra precaution of tacking down the corner stitch when i edge my 4sq blankets. it's simple.
pull the thread snug and continue stitching down the next side of the blanket
when done carefully, the tacking stitch is not noticeable.