Maybe this is pretty much a non-issue, but I thought perhaps it should be touched on. The seam allowance in any fabric construction serves some very distinct purposes and if one does not really understand the reasons for to be there it can have a negative impact on the project. Obviously, and the most intuitive, is so there is room for the stitching line without taking away from the finished project's size. If you cut the fabric exactly to the size you want you will not be able to maintain thosemeasurements once it is sewn up. That's kind of a no-brainer.
But the seam allowance is more important than just that. The width of the seam allowance is a surprisingly important part of the pattern. In the world of commercial sewing patterns, especially for garments, the seam allowance is 5/8" (16 mm) This is an industry standard which is somewhat universal. As if anything is truly universal in the garment industry. If you are drawing out the pattern to finished size and then adding seam allowance that is a rather easy thing to do. Just add another line that distance from your seam line and cut there. But for me, at least 5/8" is a cumbersome figure to use if you want to draw just the cutting line. So I will often make my own patterns allowing a 1/2" (13 mm) because it is easier to work with.
But that is as narrow as I would go. Making the seam allowance any narrower than that is asking for some trouble. First, anything narrower is not going to fit the feed dogs of most machines I am familiar with. The standard presser foot is designed to cover the feed dog area and provide good contact. The feed dogs can be rough on fabric and if they come to close to the edge the fabric can be subject to fraying as it moves through. That is not a good thing.
After the seam line is sewn a wider seam allowance provides so security against the seam fraying and failing. If the edge of the fabric ravels it will get tangled on itself to some degree if the there is enough fabric to allow it to do that. As an experiment try to strip a number of threads at the same time from the cut edge of a panel. It is mush more difficult to do that one at a time. A proper seam allowance helps keep the fabric from coming apart in the same way.
A proper seam allowance provides some structure and form to the project. If you follow commercial garment instructions you will find directions to fold and iron the seam allowance in a certain way. On flat panel seams you can iron the seam allowance flat right down the middle. But on other applications you will want to fold and iron the seam allowance to one side or the other. That changes the look of the project by moving the bulk of the faric away from one area, or by providing addition structure by placing the bulk in a certain place. Changing the seam allowance can change the way that structure affects the project.
To wide a seam allowance means added bulk and messier seam lines. It uses more fabric and adds weight. But the seam allowance is an important part of the pattern. Make sure you use it to your best advantage.