If it's "no clean" flux, I don't see why a vendor should be expected to remove it; it is rather greener to leave it on (no volatile solvents polluting anything.) OTOH, I wouldn't there to be so much left that it's an eyesore, and I'm not sure how you as the customer can know whether or not it's a non-corrosive flux... (although, it's the water-soluble fluxes that tend to be more corrosive...)