When I make these I just pin down the middle with 3 long pins when I put the top face down onto the back. I don't do much with pins but I discovered when I did classes in my home that some people need the security of pins. I like to tack when I need a pin instead when you make the Boston Blocks
I also think pins let things move too much or get in the way. If I do pin I usually place them farther back from the seam allowance because I hate to pull pins while I sew.