Sewing a perfect seam is the goal of every project. Many factors must work together in harmony to obtain the desired result. The machine and needle must be in good condition, the thread must be able to lie smoothly and evenly in the fabric, and the tension must be set properly to accommodate the thread type, size, and any differences in the top and bottom threads. We will assume the machine, needle, and tension setting are all OK and therefore focus on the thread.
The finer and smoother the thread, the better the seam will appear. Some piece with polyester because it is so smooth and free of lint. The risk of using poly is when ironing the seam and melting the polyester thread, cotton is a safer choice for piecing. Hold up some cotton piecing thread to the light and examine a few yards of it. If the thread is very fuzzy or has "slubs" which are clumps of excess lint spun into the thread, it is not a high quality thread. The amount of fuzz will affect the quality of your seam. On the other hand, if the cotton thread is extremely smooth with absolutely no fuzz, that is NOT a good thing. The only way a cotton thread can be free of fuzz is if it is waxed or glazed with a coating to cover the fuzz. Some such threads are labeled "glazed" but unfortunately, many others are not. Glazed threads are OK for hand work but are not good for any type of machine sewing. The glazed coating will rub off in the tension disk area and everywhere the thread makes contact and will gum up the machine. If you can see some fuzz on the cotton thread, it most likely is not a glazed thread.
The higher grade the cotton, the smoother the thread and the lower the fuzz. Cotton is classified by the length of the staple, or fiber. Labels on most cotton threads do not specify the staple classification because the majority of cotton thread is regular, or short staple cotton and that is nothing to brag about on the label. Long staple cotton and Extra Long Staple (ELS) cotton thread will be prominently labeled as such because they are premium threads. You will be much happier using the highest grade cotton. Although it costs more, it is well worth it. An extra long staple cotton will have very low lint and will be a much stronger thread. This means a cleaner sewing machine and less breakage, frustration, and down time.
A low grade cotton thread that is not consistently smooth and has excess lint will not create the best seam. The "slubs" and excess fuzz in the thread will create lumps in the seam. They also get caught in the tension disk and the eye of the needle and cause breakage. On the other hand, a high grade extra long staple cotton thread will create a very smooth and even seam. A fine, smooth thread does not add bulk to the seams. It creates a much flatter seam which is especially important when sewing blocks which contain a lot of points. It is easier to use, will keep your machine much cleaner, and your finished project will look much better. We spend thousands of dollars on the machine, hundreds on the fabric, and it will be well worth it to spend an extra dollar or two to get the best quality piecing thread. You and your machine will notice the difference.