Imagine walking into your house one day and in the middle of your living room is a brand new 60-inch plasma screen TV. Your husband just spent $3,000 on the biggest and best. And then he says, "This is the best ever! Great picture, stereo surround sound, and it gets only one channel all the time . . . ESPN!! Sports, all day every day." $3,000 for a one-channel TV sounds crazy. But, that is what you are doing when you use only the preset automatic tension settings on your expensive sewing machine. If you never change the auto tension settings, you are stuck on only the channels your machine likes. You are prohibiting yourself from enjoying the other channels available, such as the heavy cottons, the fine holograms, the sensitive Metallics, and other specialty threads. By overriding the auto tension settings, you can use different threads in the top and bottom. Many decorative threads require a looser upper tension setting than the auto setting provides.
By adding to or taking away from the top tension, you can equalize the strength of the two tug of war teams and create a perfect stitch. The fiber content does not need to match. The weights or sizes of the threads do not need to match, but they should be reasonably close (for example, don't use an ultra fine 60 wt. bobbin thread with a heavy 12 wt. top thread). Some educators teach that you should always use the same thread on the top and bottom. That's OK, but you're limiting your channels. It is much more fun to get all the channels and be able to use all those fancy threads. 90% of all sewing frustrations can be eliminated by using quality thread, choosing the right needle (the Topstitch 90/14 is the needle of choice for many professionals and educators) and by adjusting the upper tension (usually loosening it). Tension adjustments allow threads of different textures or weights to freely pass through the tension disks. And don't forget to clean out the tension disk area. Lint can accumulate and affect your stitches.
Learning to override your machine's automatic tension system will allow you freedom of choice in selecting those different thread channels. Don't worry about getting so far into the computer settings that you will really mess up the computer. The factory preset settings will reset themselves every time you turn off and on the machine.