Should I tak antioxidants to prevent harm from marijuana?

First I will literally answer your question.  Marijuana smoke does represent an oxidative stress and from a purely theoretical basis, it would seem that a marijuana smoker would benefit from antioxidants.  I don't know of any literature on specific antioxidants for marijuana smokers, but I would expect that taking something that boosts glutathione, a major antioxidant, would be a good idea.  Lungs of marijuana smokers show lower levels of glutathione.  Glutathione is not available as a supplement but can be obtained through Milk Thistle or in SAM-e taken with B Complex cofactors.  There are all sorts of antioxidants out there and it is unclear which other antioxidants may be helpful.

To address your question on another level, if you are that worried about the effects of marijuana, are you also concerned about memory loss, weight gain and amotivation?  If you were only an occasional user, I doubt that you would be concerned about antioxidants.  Physical health aside, there are so many negative sides to chronic, constant, marijuana use.  Is the oxidative potential really a serious focus of concern, considering all the other potential problems?  If you are going to make a list of the positive and negative aspects of marijuana use, oxidative stress is probably pretty low on the negative list.