I think farming sheep is nearly unavoidable in Minecraft. I have a corrals with cows, chickens and sheep, though these days mostly I only pay attention to the cows for leather. They're all gray though, I'd have to dye the wool. Though why the wool color matters...?
I didn't realize there was a macerator recipe for wool -> string. Because I've always found the shortage of string in Minecraft to be artificial, I installed a Flax mod before I even started GT5U. So mostly for me it would be the wool.
The wool color matters because the wool-based cable-insulation recipes specifically require black carpet. Also, remember that you can dye the sheep, and get the appropriate color of wool every time you shear them.
BTW, GT already adds a flax crop, as well as spidernip (which has string as the primary drop, and spider eyes and cobwebs as the special drops).