If you can get your hands on a copy of "Fantastic Fit for Every Body" by Gale Grigg Hazen [ISBN 0-87596-792-2 published in 1998], pages 182-185 give you a step by step procedure to fit a tank top. Gale does it with a full copy of the front and back pieces in paper with the neck and armhole seam allowances trimmed off. The pattern size is chosen so that the bust circumference is correct. Essentially you are draping the shoulders on the body. For the upper chest to lay flat, the shoulder seams will be off set from each other. You need to alter the front so that the shoulder seam will line up with the back shoulder seam. Now that the shoulders are correct, trace off the revised front pattern, pin the shoulder seams together and see how much excess fabric is in the front armhole -- the front armhole will be lower than the back armhole. The front armhole needs to be raised the amount needed to match the back armhole and the shape of the armhole redrawn. My process is to ease this extra length on the front side seam in the area where a bust dart would normally be to make the front and back side seams the same length. Gale goes on to state that for an extremely large bust, you will have to add a bust dart as well.
I have been working on getting a TNT knit top pattern these last few weeks. I definitely need some sort of bust shaping and would prefer not to have a dart. I have concluded that fabric definitely makes a difference. A fabric with vertical stretch is going to need less shaping than one that is woven or a one-way knit. Peggy Sagers, Silhouette Patterns, uses a french dart in all of her knit patterns because it is less noticeable. Peggy also reminds us that the shoulder angle is also a dart which can get rid of wrinkles in the armhole. In a tank top, that's something that is really easy to try!
I think it was on this thread http://artisanssquare.com/sg/index.php/topic,13875.msg229293.html#msg229293
that there was a discussion of how Connie Crawford's knit t-shirt pattern doesn't need darts if you make it in a moderate stretch knit. Someone reported on a class with Connie that she'd attended. The t-shirt size is chosen by the full bust and then the shoulders and neck are fitted by draping. When I've tried this, the excess fabric wanted to go towards the centre front and I didn't know how to get rid of it. But, I was watching Connie's DVD on Pattern Fitting and Truing today and she showed how to eliminate a dart formed by excess fabric in a neckline. Perhaps that will work on a knit top? In some ways, my most successful attempt has been to drape my paper tape dress form with a knit. I was able to eliminate all the wrinkles in the upper body but I did need to drape in a french dart. I have traced off a paper pattern from the knit but I've not tried it out on another knit.
Edited to add that I tried this on one of my multiple knit muslins. This test garment had no sleeves but I'd rejected it because it had too much excess fabric around the armhole and little bits wanting to be darts. I unstitched the shoulder seams and put it on my dressform. I smoothed the back up and then the front up over the shoulder. I repinned the shoulder seams. In this case they did actually meet at the neck edge but were offset at the armhole edge of the shoulder seam. I needed all the length in the back but the front was shortened about 1". When I tried the pinned changes on myself, I was amazed that this simple change removed all the problems and the garment fits smoothly above the bust. I had expected that there might be some excess fullness moved into the neckline but in this case there was not. I still have a french dart in this one but I was able to reduce its size.