An inch is an inch is an inch.
Every time I hear/see this, I keep wondering if there are knock-off rulers that are being sold that the inch is not an inch.
Perhaps those who are saying there are differences, would show us some pictures of the offending rulers, and tell us the brand names????
One thing that I do know ... I prefer using my Olfa brand rulers over the OmniGrid rulers. Because of the fine lines on the Olfa it is much easier to cut with precision. With the OmniGrid "fat yellow" markings, it is easier to cut a thread or two wider, or narrower because of the thickness of the lines. That can easily throw off piecing results.