There have been two comments that I want to address. The first was left by Chrispy commenting on Hannah's balance sketch she said...
"Ok I understand the principle but why balance them. Is it so they lay better on the body?"
This is a complicated question to answer because there are different reasons for different aspects of balance. I want to preface all of with the fact that this is just my understanding and could be wrong. In this case we have lumped together a few different ideas so I'll try to address all of them.
1: Keeping your side seams parallel.
2: Making the front larger than the back.
3: Keeping your shoulder seams parallel.
4: Aligning grain along the side seam or any seam for that matter.
I'm going to start with 2 because its easier. The front needs to be bigger so you can breath. There is also an element of "Looking right". Even though the side seam is not exactly 1/4th of the way around your body it looks like it is. The greeks were really into this idea and made whole buildings that looked "right" but weren't. The columns got bigger at the top to make them look straight for example.
As for 3, this is a way to keep your shoulder seam straight. The length of the neck line and armsye shouldn't change just the orientation of where that shoulder seam connects to the neck and armsye. When that seam is straight the shoulders are parallel as in Hannah's balance sketch. In this diagram the black would be correct where as the blue and orange would not give you parallel shoulder seams. Hopefully from this you can imagine how you could move the orientation of your shoulder seam in order to make them parallel.
Now for the biggy #1
Why keep your sides seams parallel?
This is a diagram of "how a dart is born" from one of my favorite pattern manipulation books "Basic Pattern Skills For Fashion Design" by Zamkoff and Price. I think that it does a good job of explaining how taking a dart out of the front changes the grain line on the side seam. The green line comes from when you keep your cross grain parallel to the floor across the bust and around the back. This is only possible where the darts are taken out along the princess line. The other line is how the grain gets distorted when you take a dart out on just the bottom.
This is where my question to hannah came in. It is easy to fold up the dart and keep the side seams parallel but the way that it distorts the grain lines at the side seam is a choice that you have to make as a designer. For me it will depend on the design on the fabric for instance you could match the stripes at the side seam if there were no darts. It might also depend on how I might want to distribute the ease. If you have bias grain going across an area where you might need more room you could use the bias to reduce bulk.
4: I also like to think about the side seam being not so much a break where two pattern pieces come together but as a large dart that just isn't connected anywhere. If you imagine a standard fish eye dart where it goes from skinny to wider to skinny again, it would seem weird to not have it be symmetric. But if it was symmetric you could slice it down it center line and lay the two pieces on top of each other and they would be not just parallel but identical. This is what you are aiming for when you balance your patterns.
The other comment I wanted to respond to is about the gaping at the back of Taylor's dress. liron said...
might I ask why you were worried about the side seam? I mean, why would there be a problem if you pinch the extra fabric from the center back and taper it to nothing on the side? (I am really asking as I am new to pattern making). You could also do it in more than one place-it just might give it some more balance (I think)."
I'm going to respond to this in the same post because it takes exactly the same sort of thought process to understand this issue as the balance stuff. This is not to say that its the same issue it just feels similar. so this is a sketch of the general shape of the back pattern piece of that dress.
If I just pinch out the part that's gaping I worry about the distortion it causes. This sketch is the same as the first with a dart removed at the place where its gaping. Its a little exagerated but I wanted you to be able to see it.
The issues that come up are the angle in the back line, the angle it put in the side seam, as well as general shifting of the grain along the whole top part of the pattern. My thought was to do it anyway because the amount I have to take out is so small (unlike this picture) and then just true up the pattern. It always bothers me though to feel like I'm letting things slide that could be understood so if anyone has any suggestions or knows why I should or shouldn't do it like this feel free to speak up.