Hey Mousie, in my case the Quilter's Rule has grid lines molded into the ruler itself and it has become my go-to 6.5" long ruler, I also have an O'lipfa square that is similar. You really know whether your ruler is the right side up or not because they are noticeable.
There is clear tape you can buy, or sandpaper dots, or other things. Some people have suggested rubber cement that I think I will try, the thing for it is you can position the glue right where you want it, but then you can rub it off for another project -- I like that.