Look at it this way. Every tie that you add will reduce the shifting. Tie as much as you are willing. You said you have 12 blocks but you didn't say how large the blocks were. If you tie at the corner of every block and you have a tie in the center of each block how far apart will the ties be? If the blocks are 9" the ties will be 4 1/2" apart which should be fine. If the blocks are 12" then the ties will be 6" apart which will be a heck of a lot better.