It depends, man. If there's a ventilated system (like a fan in the roof of the attic that pumps air out all day) you'd be fine, but if there's not, it's likely the air from the attic is sucked down into the house to where your AC/Heater is at (central air and heating only).

Basically, I'd say you're safe- just smoke up there late at night, and be ready for your dad to bust in with a bat thinking you're a robber.