From what I have read on the matter (not especially a lot, but much personal experience), nicotine does not have an anxiety-reducing effect, excepting those persons who are addicted to it. For me at least, if I don't chew tobacco for a good six hours, I begin to become more nervous as the initial effects of withdrawal set in.
I wouldn't be able to comment on whether or not smoking would increase/decrease anxiety disregarding the withdrawal, as quitting tobacco use (for good) is just not something I feel like committing myself to at the present time. It's extremely hard, and if there's anyone out there on the fence about getting hooked to help dissipate anxiety, I wouldn't recommend it.