There's no succinct answer to this question. It depends on where you live and where you work.
If you're going to live or work off the Metro (or both), you're going to need a car unless you can find a bus route that suits your needs or can find someone to carpool with. Even for routes that are nominally Metro accessible, you're still going to want a car a lot of the time. One reason for this would be because there's no particularly good way to get from the Metro station without a car (for instance, Greenbelt station is literally just a parking lot abutting the Beltway). Another would be because the Metro route is going to take a lot longer than driving would; e.g. 14th/U to Wiehle/Reston by Metro is 75 minutes during rush hour service according to the WMATA trip planner. I don't have a car so I can't speak to what that traffic is like but I'd be surprised driving was consistently worse than that, and you could easily do better if you have the freedom to shift your schedule to 8-4 or 10-6 instead of 9-5.
Now the Metro can be pretty dodgy in terms of reliability (just how dodgy depends on what line), so really your ideal situation would be to be able to walk or have a short Capital Bikeshare (or self-owned bike) commute. That said, Metro is typically better than driving or taking the bus as long as you're not trying to do something like Silver Spring to Bethesda (36 minutes by Metro, 14 minutes by car without traffic).
The other thing you have to take into account is that Metro fares are distance based, and go up during rush hour. So for example, I live near U St and work in Pentagon City. I usually work 10-5 or 10-6 so my commute is $2.20 in the morning and $2.75 in the evening. Let's say I lived up by Greenbelt to save money, that's be $3.60 in the morning and $5.65 in the evening. That's an extra $86 of month on Metro; another $102 for the daily parking at the station; and that's not including all the extra money I'd spend on a car (gas, insurance, etc). I definitely pay out the ass for rent but it largely evens out by virtue of not having a car. And in general around here, when you look at rent, you can't just look at rent, you have to look at rent+transportation costs. If you just looked at rent you'd go out to Vienna or whatever in search of "deals" but then set yourself up for a miserable and probably not-cheap commute.