In places where pot is legal for recreational use, I would assume it has the same stipulations as alcohol and you wouldn't show up drunk. I am assuming if the employer does random drug test that there is leeway to how much is in your system. After all it is just as legal in two states for recreational use just as alcohol is. For states that prescribe it for medicinal use, you have to have a prescription and I really don't know how employers react with THC showing up on random drug tests.
The federal government does not recognize either the recreational legalization or medicinal legalization. So if your job subjects you to federal guidelines and laws and regulations, THC showing up on randoms over the threshold is a no no, prescription or if you live in a state it is legal. People that live in Colorado and Washington state if they drive trucks, fly planes, run trains, work at the post office and so on it is still illegal to use. State workers in the states where it is legal, I really don't know. Unless you are politician and then you have no rules to go by.

Also if you are caught with it in a national park say in Washington state by a park ranger, you can be busted for posession because that national park is actually federal government land.
I know I hijacked the thread a little but all of what I said may keep people out of trouble.