To get a straight edge, I started with a row of triangles along the bottom. For sweaters, this had the advantage of pulling in the bottom edge. For an afghan, I might make the block fabric first, with the jagged edges, then fill in the edges afterward.
Annie's Attic had afghan patterns using the entrelac technique. One really interesting one used a variety of stitches from single all the way to trebles, so the blocks were not square but formed scallops. Annie did a lot of really creative crocheting.