The rounded corners are fine. Just don't join right to the corner just go as far as you can so it lies flat and you'll have a four sided star where the corners join.

I suggest you sort your squares into large and small squares. Lay out all the small ones into a square or rectangle whichever your finished afghan is going to be and then do the larger ones on the outside. It might help if you lay them all out on the floor like a finished afghan. When you have a good arrangement take each row and pile them one on top of the other now string them together with a strand of yarn and attach a piece of paper with row 1 on it or use different colored wool for each row and then mark color and row number on a paper.

This way you'll know exactly which row goes where. Sew one row together and then another and then sew the 2 long stripos together

I'm making granny squares too and I have been trying different methods. Many of them look bad on the bad, or do not lie flat.

I like this one.

Here is a link that might be useful: InvisibleGrannySquareSeams

Thanks for all this info. so helpful. So I took out the sc around the edge and just made the squares bigger with the purple color. Then I did put the small ones in the middle in a rectangular shape, and attached them with the whipstitch which I think it what you described above. Then I attached the other ones just with regular sewing I guess--much quicker and stronger and better since they were not all the same size. didn't really show since it was still purple. Then I did a scalloped edge. I did a lot of blocking. I would like to put a picture of it but don't know how.

Thanks for all this info. so helpful. So I took out the sc around the edge and just made the squares bigger with the purple color. Then I did put the small ones in the middle in a rectangular shape, and attached them with the whipstitch which I think it what you described above. Then I attached the other ones just with regular sewing I guess--much quicker and stronger and better since they were not all the same size. didn't really show since it was still purple. Then I did a scalloped edge. I did a lot of blocking. I would like to put a picture of it but don't know how.