Starching helps keep the fabric stable.
Another problem for some is they press the seams by ironing rather than pressing and then lifting the iron and move on. Most irons have a curved side which we use the push the fabric away from the seam and this curve of the iron can cause some distortion if you're not careful. Setting the seam (press the seam from the wrong side before you move the seam allowance to one side helps as well on the pressing issue.
The way your material feeds through the machine (feed dogs, sewer pulling/pushing etc) can also result in some distortion. The shorter the strip the less the issue here.
Cutting can also be the culprit. Things that are not cut on the straight of grain are more likely to distort. It is difficult to get a good true straight of grain on long pieces with cutting. The less volume of fabric you're working with the easier it is to manage and fold it and position your ruler in such a way to try to get it as straight of grain as possible.