Marcia, did you check the width of your strip sets before proceeding? That's how you would tell if you need to change your seam allowance. Keep trying until the strip sets come out exactly the right width. Once you've established your seam allowance, make about five sample 4-patches to make sure things are coming out right. It can take quite a while getting the seam allowance right, but never was time better spent.
As for pressing, I take the sewn 4-patches to the ironing board, set all the seams, spin the seam, touch the point of the iron to the little 4-patch on the back, then turn over and press carefully. I used to push too hard against my seam lines with the iron and distort my blocks. Use a light hand with your iron. Also, consider that photos you've seen may be showing units that have been squared up.
I couldn't resist starting a second mystery quilt in Bonnie's colors. This isn't sensible, since I've never been able to keep up with only one quilt, much less two. I have a single fabric for the blue, a single green, three tonal neutrals, and a few reds and oranges. It took just a fraction of the time to press and cut the yardage, compared to the scrappy. I'm currently pressing my #1 4-patches, and ready to sew the #2 4-patches. Whatever stage I'm at by morning, I intend to start right in on clue 2.