SHOULD I TELL THE TRUTH? – Look, as a lawyer, just as anyone in an official capacity, the only advice I can give you is, tell the truth. There’s a lot of jibber-jabber here but I think you get the idea. Be informed.
“If you’re asked if you smoked marijuana, that is not a question that you are obliged to answer at a U.S. port of entry.”
Source: Do not tell U.S. border guards that you’ve smoked pot, despite what some say: lawyer