If it's something that I am doing for a "serious" painting as opposed to a for-fun/learning/derping around painting, I always plan the line art on tracing paper first and then I transfer it to the canvas/board/whatever when I am done. That way I know my lines are where I want them to be and I won't wreck the surface of the canvas or board with eraser streaks. I want it to be done as clean as possible. On my most recent painting though I planned the line art out in Photoshop and thought it might be quicker to grid and transfer directly onto the canvas; bad idea. I thought it might be faster but it absolutely was not. /sigh.
It's totally worth it though, in the end Tbh I am lazy as well, transferring is the worst part of the process. But your end result is a lot better. ^__^