Does it matter which wrist I wear my watch on? Do I need to change settings accordingly?

There is a wrist setting and it is supposed to adjust the gesture recognition for backlighting. I don't think it affects anything else.

But any step counting watch should be worn on your non dominant hand or you'll get loads of steps counted for things like brushing your teeth etc..

Luckily the algorithms of the Fenix are more advanced - at least my toothbrushing is not counted as steps.

Not sure, how advanced they are. Everymorning, I reach my step and floor goal just sitting in the bus going to work.

That is strange. I commute to work by bicycle, local train and bus (30km), when I arrive at work I have around 1500 steps on my F5+, which is quite correct. It does not count a significant amount of steps during cycling (MTB, Road) and driving a car!.

I just hit my 5000 steps goal while taking a shower. By the time I'd finished washing and drying the steps count was at 5237. So I'd guess roughly 500 steps added just for showering. I also cleared my move bar as I stepped out of the shower.

Not really that smart.

Of course, it probably would not have mattered much which wrist the watch was on for showering, but it should be the one that you swing only/mostly when you walk rather than brushing teeth, washing the car, sawing logs,  polishing furniture, troweling the garden, flicking a table tennis bat, tapping on your tablet/phone, taking a personal moment etc.. ;-)

I would say that taking a shower burns about the same calories than taking 237 steps. Perhaps more. These devices are meant to give you a very rough idea of your activities and fitness level, even if they are marketed as precision instruments, which they are in no way at all. "Steps" is just a way to express energy spent on a physical activity, so it's perfectly fine for me if my watch counts steps while I am taking a shower...