There are a few hotly contested things in the quilting world, but none more so than pressing seams open vs. to the side. If there's anything that the quilt police love to get on their soap box about, it's this.
But . . . pressing your seams open is good for a lot of things, including precise piecing. Here are 3 reasons why:
1. Pressing seams to the side unevenly consumes fabric in your seam allowance
Fabric is 3D. Folding 'eats up' fabric so if you're folding the seam allowance, but not the other, one piece is going to be shorter than the other.
Below is an exaggerated illustration of this effect. My folded 6" piece of fleece is not 3" anymore. It's almost 1/8" less than that due to the amount of fabric "eaten up" in the fold.
This exact same thing happens when you press your seams, but when you press a seam to the side, the key is that it's happening unevenly. That means that your fabric pieces aren't shrinking consistently and that will throw your finished piece sizes off ultimately affecting your ability to match points and make a precise quilt.
You might think the difference is so small it's almost imperceptible. That's true on a single-seam basis, but it adds up significantly over a whole quilt. (Read more about this here.) You WILL notice a difference when you switch to open seams.
2. Pressing seams to the side increases the likelihood of having a mispress incident
A mispress is when the seam allowances don't fold exactly at the seam. Here is an exaggerated picture of what that looks like.
Like paper, fabric has some resistance to folding. If you're folding one fabric but not the other, the stiffness of the flat seam allowance is going to want to push the seam underneath the fold of the folded seam allowance resulting in a mispress. There is a technique that has to be developed to keep this from happening. Hence why most "to the siders" set their seams with an iron first and then use the iron to push the top fabric away from them. It's not impossible to do it right, but it takes a little practice.
When you press your seams open, each fabric resists folding equally but pulling in opposite directions. This makes it very difficult to end up with a seam that isn't smack dab in the middle of the two pieces of fabric which is what you want. It takes the finesse out of pressing and that is a very good thing.
3. Open seams 'lock' your fabric edges so your seams don't fray
If you've ever had a problem with your quit seams popping open after you've been using your quilt for a little while, it's probably because you're pressing your seams to the side.
Quilting fabric is woven. That means that it's highly susceptible to fraying and can almost 'dissolve' back into its constituent threads at the extreme end of this spectrum.
Fraying can occur due to many things, but one of the things that causes it super fast is pulling on the threads near the edge of the fabric . . . like when you pull on a seam near the edge of the fabric. In order for the fabric to fray, the interior threads need to move out to the edge of the fabric.
When you have a seam pressed to the side and you pull on the adjacent fabric, that's exactly the effect it has. The edge of the fabric is in the same direction that the adjacent fabric is being pulled and over time, the seam allowance will fray and allow the stitches in your seam to pop loose. Not good!
When you press your seam open and then pull on an adjacent fabric, the force is opposite to the edge of your fabric so any distortion that occurs isn't going to travel to the edge of the fabric and fray the seam. They're effectively locked.
The caveat here is that now the thing that determines if your seam pops is up to the strength of the thread used in the seam. If you're using cheap, weak thread, the thread itself can break ruining your seam. That's why it's important to buy and use good quality thread.
The good news is that good thread is in abundance these days. Any reputable brand will provide plenty of integrity to stand up to the beating that well-loved quilts receive. (Read more about choosing a good thread here.) Making sure there is an adequate amount of stitching across and next to the seams will also help reinforce these seams.