Sorry, I was around just taking care of this green lady and beaming down some red shirts. I'd say if you're not having issues with rotary strike and what you're trying to accomplish, then don't think twice about it. If one day your machine is running sweet, and the next time you hook it up and you can't get that same saturation, barring damaged or clogged needles...polarity mixed with the technique weakness posted earlier might be to the cause.

One builder earlier mentioned brushes and brush-less motors. Brushes really only like to spin in one direction because they will get uneven wear on the brushes, one edge will be low towards the main spinning direction and the other side of the brush will wear high and sharp, when you reverse the spin of a worn brush, the high sharp brush edge will catch on the contact surface of the motor. Much like a chisels bevel does on a piece of wood. What that does is make the motor work harder, to overcome the worn sharp edge of the brush when it spins, it also makes it run slightly out of it's harmonic balance.

Brush-less motors do not have this problem, however they do have a magnetic North-South running preference, running them against this polarity doesn't make too much difference in motor life my guess 15% less, unless it's bridged with a diode. A diode is supposed to only allow electricity flow in one direction. Unintentional flip flopping not knowing any polarity, machine and or clip-cord the motor will die a lot sooner.

Running the motor in the same direction is key for both motors and it's life. However, if running opposite the polarity makes for an improvement in your work, then always run it reverse (unless of course the machine is bridged with a diode) contact the maker to know for sure, or find a digital multi-meter and test it yourself. Never ever keep flip flopping any machine not knowing any polarity unless you want to kill it. Coils run in a looped circuit, where polarity matters as well, if your coils heat up very fast or you keep popping capacitors you need to check your polarity.