I have never pressed the seam open so that part???????
When you do regular sewing, you usually put right sides together and then your seams end up on the back. For a rag quilt, you place your pieces, or blocks wrong side together so that your seams will show on the right side of your quilt. I have never "pre-snipped. For your sewing you probably want at least a 1/2 inch seam or even an inch, set your precut/snipped blocks wrong sides together and sew. Make sure that your seam is below your snips. When you go back an snip the seams of non- pre-snipped blocks make sure that you do not snip into the seams.
When you are done with sewing it all together and snipping all the seams you throw it through the washer and dryer and then it will rag up for you. Leaves a lot of lint in your dryer!!!