When making straight cuts you also have to remember the width of your ruler matters too. I fold my fabric twice so I have four layers. I line up the top of the ruler at 1/2" and the bottom must line up on any line at the bottom. If both don't line up then the fabric isn't folded straight. Also if the ruler is only 6" wide that is all you've squared up. Ex: you want to cut 8 2" strips. The first three cuts are square if you started correctly. Then move the ruler so the whole ruler is on the fabric and trim the cut edge after you line up the ruler at top and bottom. Then you can make three more strips. What I'm trying to say (I sure I've made it as clear as mud) is the wider the ruler the more cuts you can make without double checking the cut edge. Most of the time I use my 12 1/2" square ruler so I can make more cuts before squaring my cutting edge another time. Hope I've helped and not made you say "what is she talking about?" Later today I'll see if I can find the tutorial. I think it's the one Alex Anderson did.
Oh, and to answer you're question I use my 60mm most. Have all sizes but like it the best.