Tobacco is not as addicting as you think it is. Mixing it in a joint has a very little chance of becoming habit forming for someone. I smoke cigarettes all the time but I don't consider myself addicted because I can go for several days without one.

As for your question, I'll mix the two if I don't have a lot of weed, just to fatten it up a little more, but if I'm rolling a mega-fatty I wont mix.