Whatever your tension, you want a balance stitch. A perfectly balanced stitch will have the top thread and the bobbin thread meeting in the middle of your work. You don't want the top thread just laying on top of your sewn fabric and you also don't want your bobbin thread just laying on the bottom of your fabric.
I adjust my bobbin tension myself all the time. Just either note the position of the screw or take a picture so you can return it to the previous position. When I FMQ I usually have to adjust the bobbin a little tighter of looser depending on how the stitches look on my practice piece.
The machine has to go to the technician if the timing is off or if you are having a problem. Opening the body of the machine will void your warranty. I clean my bobbin area myself with the little brush that comes with my machine. I have a little oil can that comes up on the computer screen when it is time to put a drop of oil on the bobbin hook. If I brought my machine in every time I wanted to adjust my bobbin tension, my machine would always be in the shop.