It depends on where you are at as a driver. If you are struggling to get around the track without hitting things every other corner, I would stick to one car and work on a stable (pushy) setup and get clean laps in, then gradually start add steering to your car.
If you drive pretty well and are looking to just get faster, driving other car types can help. If it's 1/12 make it a fast 1/12, 17.5 speed 1/12th can give you bad habits, like never lifting and just cranking the wheel around corners. Even in slow tc classes you have to learn to be smooth and "feel" the weight transfer and time your steering accordingly. (yes a lot harder to do than it may sound).