I always trim mine. There are plenty of references on the intergoogle to help you learn how to trim. Basically though, remove the meat flap on the underside, remove the membrane, cut as close as you can to the top of the longest rib and then follow the ends of the ribs as you cut across.
On a side not about the trimmings, last weekend I trimmed and smoked 3 slabs. I took the trimmings and cut out all of the cartilage. I think ground up the meat that was left and smoked it like a fatty. It was a little dry as it probably needed more fat to keep it moist. Other people will use the trimmings for their beans, chili, sausage. Its still good meat, I just don't like cooking them as riblets and as mentioned above, usually have issues with the pieces cooking properly.