I just finished making a twin size, granny square blanket, and I wanted to share a couple things I like to do when edging blankets that I think improve the overall product.
It's important to know that double crochet, when worked in the round, requires 12 increases every round to lie flat. Granny squares have 16 increases every round. That means that if you make granny squares bigger than a certain number of rounds, they stop being squares and start ruffling a little or being extra pointy if forced to lie flat. When assembled into a blanket, those extra increases have a slightly cumulative effect, meaning that the individual squares looked square, but the finished blanket isn't quite rectangular or square and might not lie quite flat.
When I make the edging on a granny square blanket, I like to mitigate some of those extra increases.
First, I work one round of (sc1, ch1), working (sc1, ch1, sc1) or (ch1, sc1, ch1) into the corner loops. In addition to working on the increases, this round firms up the edges.
Next, I work a round of double crochet. At the corners, I like to continue the visual effect of the diagonal lines creates by the increase points, but with fewer increases. At each corner of the edging, I work (dc1, ch2, dc1) in the corner stitch.
I usually repeat those two rounds a couple times, and then finish the whole thing off with a round of crab stitch or light crab stitch.
In addition to evening out the increases in the blanket, doing this edging using different colors for the double crochet and (sc1, ch1) rounds creates a nice dotted look that I really like. Creating dots by alternating rounds of double crochet and (sc1, ch1) in contrasting colors looks really nice on lots of different projects and can be used to mimic the look of fairisle knitting.