It's true!
finishing can make all the difference. My favorite finishing technique and the one I use almost exclusively (that means "all the time" unless a pattern specifically recommends another method!) is the mattress stitch. I have no idea why this seaming process is called the "mattress stitch"; if any of you know, please leave a comment!
Before you begin seaming, you should block all of your pieces so the selvage edge is easily accessible. I usually use a steamer to block my garments, but some folks swear by wet blocking, too, so use whichever method you like best.
I usually seam a garment with the same yarn I use to knit it. However, if the yarn used in the knitting is not quite suitable for seaming (like a novelty yarn, a yarn that pulls apart easily, or a bulky yarn) you might need to use something else. If you're working on a lightweight piece and need thinner yarn for seaming, try using embroidery thread. The color range is endless and the thread is really strong. Some folks also use one or two plies of the garment yarn to seam, which I also think is a great idea.
This joining method gives the neatest and most professional finish to a garment.
One secret is always working mattress stitch from the same vertical column. Never veer to the right or left of the seaming line—the stitch adjacent to the seam should be fully visible from base to top along the whole seam. If you veer into that stitch or farther away from it as you seam, your seam will zigzag in a distracting way. When seaming a sleeve into an armhole, make sure to follow this rule on the body pieces.
Another secret of good mattress stitching is to keep the seam elastic without letting it stretch too much. The best way to do this is to work the mattress stitch loosely for one or two cms, then pull the thread very firmly so that the stitches are held together quite tightly. Now stretch this seam slightly to give the required amount of elasticity, and then continue with the next section of the seam. If you are accustomed to sewing your knitting together by other methods, it may take a little while to get used to mattress stitch, but practise makes perfect and the professional finish it gives makes it worthwhile.
The most important step to making perfect seams comes after the sewing is done. I know you're dying to show off your new sweater, but investing just a few more minutes will make everything look absolutely professional. After weaving in all the ends, you might notice that the seams of your sweater are a bit bulky, especially when compared to the smooth, sleek fabric of the stockinette portions. Reblocking the finished sweater for the sole purpose of flattening the seams is time consuming and unnecessary. Instead, moisten the seams by spraying them with a water bottle or by laying a damp towel on top of them and then smooth the fabric by patting it down with your fingers. You may also lightly steam the seams to achieve the same result, taking the utmost care if your sweater is wool.
One advantage of mattress stitch is that it can be used to sew shaped edges together quite easily. Because you are working on the right side of the work all the time, it is much easier to see where you are and to keep the seam neat. Mattress stitch works as well on ribbing and moss stitch as it does on stockinette stitch, garter stitch, seed stitch, and whatever stitch you're working with!.