A square ruler will always be a square so it won't matter what one you use. Where it matters is how wide your fabric is so the size you have reaches the the other side of the tube. If you have a four strip tube use a bigger ruler , a smaller strip use a smaller ruler. It's just for convience.

Here is another tut on the same thing
http://www.youtube.com/watch?v=kVd9CfUK5gU