Bobbin Tension - A Personal Viewpoint
There is a lot of controversy about the need to adjust the bobbin tension. My viewpoint comes from 50 years of sewing myself, some of it professionally. In addition I have spent 35 years in and around professional stitchers of all types. My wife has made a living sewing professionally making custom garments such a wedding dresses, suits and tuxedos. Combining our experience we have agreed that while there are times when adjusting the bobbin tension makes sense, most of the time it is either not needed or an obstacle to proper machine operation.
Background: Unless you have the specs and the equipment to measure it, bobbin adjustment is inaccurate at best. Dangling the bobbin case is a helpful tool, but the range at which it is "properly" tensioned is pretty vague and inexact. Most manufacturers warn against bobbin adjustment of their home machines. This is not because the process can cause damage to the machine, but rather because for the most part it is not needed if the machine is threaded and maintained correctly. More on this later as gear making is beyond the expectation of most home machine manufacturers.
Why not to: The top thread tension is made to cover a wide range of applications and situations. In most cases of major sititching problems the top thread tension should provide enough range to at least diagnose the problem with a stitch line _if_ the problem comes from a tension problem.
Increasing/decreasing the bobbin tension may solve the symptom but the danger here is you may also cover up an internal problem which will only get worse. Particularly with older machines of unknown history there are burrs and spurs and grooves and etchings that occur inside the machine. These can raise a great deal of problems with tension that can not be resolved by the usual tension adjustments. Simple normal corrosion on the hook caused by normal humidity and constant temperature fluctuations over time can alter the way the machine sews. While adjusting the bobbin tension might resolve the symptom, it will not remove the corrosion or burrs/spurs which come by normal use. Removing the symptom allows the the problem to continue to build until it is beyond simple polishing and buffing to repair. For some machines that means replacing parts that may not be available any longer. You then have a nice paperweight.
The only thing the bobbin tension affects is the fine quality of the stitch. For major adjustments I would look for other causes before I relied on the bobbin tension adjustment.
When it makes sense: If you know the machine has been properly maintained and you have a history of good results then there are certain times when adjusting the bobbin tension makes sense. If you have changed the weight of the thread and the top tension needs to be adjusted into the extreme ends of the range, either high or low, then you might want to alter the bobbin tension to fine tune the quality of the stitch. The machine normally works best if the top thread tension can be set about the middle. High or low extremes do not give you flexibility you need to get a really nice stitch. But please note... this a fine adjustment. If you are still having major looping problems and dropped stitches at the extreme ends then there is something wrong with the tensioning system for the top thread. This is particularly true if the problem arises suddenly in the middle of a project.
For fine adjustments to the stitch quality adjusting the bobbin tension might be the way to go. For major problems, you run the risk of concealing problems that need professional attention. Of course those decisions are up to your personal comfort and choice.