I hate to chime in since I simply agree with everyone - this is one of the first "rules of thumb" I was taught in my cooking experience. Living in PA, we don't have very convenient access to real wine. Sale of wine and liquor is controlled by the state and only available for purchase at state stores. You'd think there'd be one near each grocery store out of convenience, but that's the thing about government-run retail monopolies - customer convenience doesn't mean squat.
The alternative is to buy that "cooking wine," loaded with all kinds of nasty stuff, mainly to make it undrinkable so it can be sold at grocery stores in states - like PA - where grocery stores aren't allowed to sell wine. I never let myself get tempted into taking this shortcut.
Thankfully, I live close enough to Maryland where I always stock up on some super cheap, but very much drinkable (and sometimes even delicious) sub $10 and $5 wines.
I've found that the difference in varietals (cab sauv, pinot noir, merlot, etc) is not very noticeable in a final dish - I've even substituted red for white wine in recipes that call for it with quite a bit of success.