I don't know if national legalization would change employment MJ screening. I think some places would drop it, but many would keep it. Drug screening is driven in large part by workers' comp and liability insurance. They get to deny coverage if the person tests positive and I'm sure MJ is far and away the primary drug they find due to its long dwell time in fatty tissue. I bet it saves them a ton in what are completely legitimate workplace injuries that have nothing to do with drug use.