On smaller jobs, it puts bidders in a difficult bind. You can't get the AISC Certified Fabricators interested in a few trickloads of steel, so they won't give prices. Then the contractor ends up finding a smaller shop, getting his numbers and pressing on. In a way, you might end up with a less quality project (fabricators who understand AISC certification but don't have it might not respond at all), so I'd leave it out.