I think it's perfectly fine to give the random drug screening assuming the proper contracts were signed. It'd show that "yeah, you mean it" when you say you don't want people smoking pot while on shift. Just giving a lecture makes it out that you'll try to just use verbal intimidation but not necessarily actually do anything to try to catch them. So you'll have some pissed at you, so what? There'll probably be plenty who'll be happy you did that cause they know that you're going to keep the place drug free, which means that they won't have to pull extra hard to make up the slack if someone gets stoned and is a lazy stoner.