I sort warms from cools because I don't care for them together, in most cases. I think the warms turn a bright thing dull.
For string blocks, 2.5" is pretty wide. You wouldn't get too many strips on a block. A variety of widths is more visually appealing, and of course anything too skinny will be very skinny after seam allowances. That said, I tend to
cut my strips from 1.25 to 2.25" wide. But if you've already cut a lot into 2.5" strips, then by all means find a jelly roll pattern that you like.
One more thing: there's an advantage to cutting various widths--you don't have to worry about matching points.