Upload & Sell: Off
You definitely don't need an external meter with that system.
First, ETTL is fairly good. It can get you close (if not on the money.) It's going to be much faster to use ETTL and to review a few test shots than to use a meter. The trick is in the reference. Many people don't realize this but you can easily use a gray card to set your exposure with flash. All you need to do is to take a properly exposed picture of your gray card, zoom into it, and see where it appears in the histogram.
Personally I prefer a bright reference for this...not white, but brighter than 18% gray. The reason is that even a change in exposure of 1/3 EV stop is seen easily in the histogram of bright objects. I use a white-balance reference called the Digital Gray Card by RMI, which is about 30% gray.
So here's how you prepare (you only need to do this once.) Mount your flash on your camera, set P mode, make sure E-TTL II is enabled, place your reference against a wall, step back, and take a picture. Your camera, in E-TTL II mode, will use distance to set the flash power (with an adjustment for ambient light,) and that produces the most accurate exposure. Now review the image, zoom in to the reference, and review the histogram. The histogram should be just a spike representing the reference. Remember its location on the histogram.
Now, setting exposure should be very easy, whether it's ambient or flash exposure. With ambient light just take a picture of the scene with the reference in it, zoom into the reference, and then adjust Exposure Compensation by some amount to put the spike in the right place. You'll soon get a feel for how much the spike moves with changes to EC, and after a little practice you should be able to set a perfect exposure with one or two tries.
For flash you do the exact same thing. You simply take a flash exposure of the scene with the reference in it, review the histogram of the reference, adjust the transmitter EC, and you're done. The beauty of this system is that you can set up the lights quickly, get correct exposure quickly, and if you want to change the positions of the lights, you can likely do so without having to mess with the flash power again. This is so because the exposure is based on an evaluation of the scene by the camera, and the camera position and scene are still the same. So the camera will adjust the flash power to provide the same exposure. That's the magic of ETTL! Want to move the camera location? No problem...hit your FE Lock function and you can now put the camera anywhere...the camera will not evaluate flash exposure from its new position.
Of course, it's possible to set the power of the flash units manually via the transmitter, if you really feel the need to make life a little harder than necessary. All that means is that your starting point for adjusting will likely be way off. But the process for correcting exposure is still the same...shoot the reference, check the histogram, adjust until the spike is in the right place.
The modeling light doesn't last long, so it's difficult to use for evaluation. I use it primarily to perfectly place the shadows of objects. I'll move the object while the modeling light is on. It's only a couple seconds, but it's enough to quickly zero in on the perfect placement. I find it's better than chimping an endless series of images when you're trying to put something in just the right spot.