What matter more for the step count is which arm you put the watch on since most people use their dominant hand more for everything like cooking, brushing teeth etc. and that will generate steps.
The setting is probably more for other activities that identifies certain type of movements and exercises.
To prevent the watch from counting steps while you are driving you can start an activity that doesn't count steps like cycling. My method is to drive less and run and bike more :-)
The setting of the hand is there especially for the gesture for turning on the light. As e7andy wrote, it is not used for steps. As he suggests, start an activity suppressing the counting of steps, preferably an indoor activity profile or an activity profile with disabled GPS, to avoid draining the battery and filling the memory by driving. Some watches have directly the Driving Activity, on others you can use for example Indoor Cycling. Or you can create a custom activity based on one of those that suppress the counting of steps.
