Timber,
Heres my thoughts and some of my experience with the teamsters. My experience is from a previous job with Sysco where we were teamsters,
Protection from the teamsters will basically be good for you. Lets say pre teamsters, managers having a bad day. You looked at him wrong and he says you fired. What options do you have? None you go find another job and/or collect unemployment.
With the teamsters this same situation would be that you file grievance. Your shop steward. Then from there it gets grieved and you go to arbitration. They find in favor of you due to the unethical activities that the manager did. You now get your job back, plus all back pay that you lost do to the circumstances.
I am not sure about the member dues if on strike.
The chances of a strike are slim. Yes they do happen but UPS's last strike was in 1992 I think. If you do strike you will not get paid, however you are striking because you are looking for better conditions than what you currently have. UPS just signed their contract for 5 years. So once you get a contract you will be set.
Here a couple sites that may help with some of your uncertainty.
www.teamsters.org
http://freight2008.org/news.php
I am definitley for being a teamster. Yeah there is some goofy things that go on but, my opinion is we getter better things than non union. IE, benfits,pay,pension etc....
I am sure more people will respond there are a few of us on here.