I began using both interchangably. Then I began to notice that not all the blocks were the same. Just a smidgen off, but when it is multiplied over the quilt, it does and can add up to distortion. Mine did. It was more eveident with a block exchange.
I have turned my mat over and use the ruler to measure. I have the same company's ruler in a small square, and long ruler. I have cheked them together and they match. (When I compared to another small ruler of another company, there was a difference in their grid.)
I am not having the problems I was having. My points match easier, and I have better consistancy in my measures.
But if it works for you-- Don't fix it!:)