Nothing about nutrition is simple.
I was intrigued by the Observatory column in the New York Times last week. USDA researchers showed that supermarket spinach stored under continuous fluorescent light retained more vitamins than spinach stored in the dark for at least 9 days. Their hypothesis: the light promotes continued photosynthesis and protects against degradation.
I was curious to know whether they measured vitamin C. I checked the article in the Journal of Agricultural and Food Chemistry (February 2010, DOI: 10.1021/jf903596v). Indeed they did. This seems odd because this vitamin is well known to be degraded by light. That is why orange juice is usually stored in opaque containers. One explanation might be that orange juice is stored a lot longer than 9 days.
Cooking also destroys vitamin C. While I was looking for that article I came across this one, which describes experiments looking at the effects of common cooking practices (boiling, microwaving, and steaming) on beneficial antioxidants and phytochemicals in Brussels sprouts.
Steaming increased phytochemicals in fresh and frozen sprouts. Boiling did too, but only in the fresh vegetables. Cooking reduced phytochemical content in frozen samples. Microwaving was the best cooking method for retaining color and vitamin activity. As expected, all cooking methods destroyed vitamin C.
So what to make of this? Eat a mixture of cooked and uncooked vegetables and the vitamins will take care of themselves. If you do cook, steaming is great and microwaving is better for preserving vitamin activity. For vitamin C, raw wins every time.