here are the directions from the pattern
Block piece or steam-press the edges with an iron on the wool setting to prevent the edges from rolling.
Using corresponding color yarn and tapestry needle, sew the sides together, right sides facing.
Do I need to block the piece before I felt it? Felting is the next step.
It's probably having you block before seaming just to make absolutely sure you get it even.
My opinion...if it isn't curling much or if you can easily see to do the seaming without blocking then I would go ahead and skip it.