Never. As much as I strongly disagree with marijuana prohibition, I'll respect the law while it still exists.
Do you obey the speed limit, too?
Generally, yes. But, in any case, going a few mph over the speed limit is nowhere near as bad of a 'breach of law' as smoking marijuana is, so that's not a very good analogy.
Well that depends on the jurisdiction you do it in.
I don't live in CO, WA, AK, OR, or DC (legal marijuana states). I'm commenting on the laws in my own area, not laws in some other part of the country.