I inherited several rulers and such. The ones I use regularly are the 6"x24" and the big 16" square one that is marked in 1/4" increments. I also use the 6" square one, but not as often. I did not like the yellow printed one although I can understand how it would be useful on dark fabrics.
The only time I use the grid on my cutting mat is when I'm cutting something wider than 6", such as fabric for a tote lining.