I think it depends on the pattern you pick to use your scraps. If it is a log cabin, then you need to separate into lights and darks, same with several other patterns. If it is a string, then it doesn't matter but needs a center strip that is the same for all the blocks to unify it, but it doesn't have to be the same exact fabric and can be scrappy as well. Meaning if your choose the center strip to be red it can be different reds but still all red strips. In the case of the scrappy trip around the world you don't have to separate into lights and darks, you use need to have the same widths, 2 1/2" sixteen inch lengths is good. But if you have really small scraps you could do a crumb quilt and then anything goes. Lots and Lots of options. Just go for it, it is truely the most fun type of quilt to make. Using up your scraps is a "good thing"