Either way works fine mate. If you want to paint scratches on, that's the way you should go. The only reason I ever damage decals (deliberately
) is when they need to have natural metal showing through. Metalizers don't brush well at all and the damage shows what's already been sprayed on. One needs to remember where the metal shows through the paint and damage the decal 'in situ' on the model in the same places. Photos really help here. I only ever did this on aircraft that have the metalizer coat under the paint. On anything else...I would paint scratches on too. It's easier and less risky.