I've always just used my 6-1/2 inch square ruler to square up HST's. It seems to work fine for me - line up the diagonal line and swipe two sides, turn the block 180 degrees and do the same thing. But now that I looked at the video for the bloc lock, I can see that it might be helpful to keep the ruler from slipping.
Honestly, there are so many different rulers out there that it's difficult to know which ones would really be helpful and which are just a waste of money. I tend to not buy into the gadget spiel most of the time, but once in a while I find one that makes a big difference in time spent on blocks and accuracy. This might be the next one that makes its way into my studio!!