Let's admit it right up front. Blocking of lace is just a fact of yarn. Both knit and crochet lace must be blocked to make all the holes look, well, holey and so that the stitches line up and flow with good looking smoothness.
Many lace patterns are designed with a center spine from which the pattern increases. Often this results in symmetrical sides. That repetition makes blocking quite easy. Well, easy in that only half the space is needed to block the thing out.
When blocking a center spine shawl it is a grand idea to have the sides come out that same size. That is why I fold on the spine, pin/wire that line first and then match up the two layers so that everything is the same.
How do you know when your stitches are all lined up and ready to be pinned or put on blocking wires?
When it look friggin' good. That's when.
Lots of patting myself on the back over this one.