I like this question. It's one I usually answer circumstantially. For example, if we're drinking white wine, and I need a splash to cook with, chances are I snitch from from the bottle we're savoring.
If you'll be drinking, simply cook with what you'll be drinking, especially if the required amount is small. (The exception might be a chardonnay with lots of wood or smoke flavor, whose characteristics can take over. )
I tend to be a little more selective with red wine. Anything with much tannin can become very intense if reduced. I prefer fruitier reds that fall in the middle of the flavor spectrum. (merlot, shiraz, chianti, etc.)
If I'm buying specifically for cooking, I will usually pick up a blended wine. Usually it will just say "Red" on the label, this is made from a mix of grapes and is typically designed to please everybody - which makes for good cooking wine. ( I dig the Coppola "Rosso" and "Bianco")
These blended wines are usually pretty cheap too, also a good trait in a cooking wine. However, I'd like to note that if you're going to make a wine reduction sauce, not to go too low. Because reducing a poor quality wine only concentrates the poor quality.
