I am a libertarian, so of course my opinion is legalize it. I don't do it, but who am I to say what others can do or not do.
As for drug testing welfare participants, it costs a lot more to drug test than it is to just let people take the welfare. Now, I think it is unfortunate when people become dependent and then just go on welfare, however, if we are thinking about the fiscal side, it is much smarter to just let them have it when weighing out the costs.