A good watch performs consistently for a wide range of its power reserve, but most run slow when low on power.
To get a consistent performance, you should wind an automatic watch FULLY from a dead start before wearing it. It takes only a minute. This habit ensures that your watch is always at or near its peak power. The watch will then run consistently, and will be less likely to stop in less time than its maximum power reserve (typically 48 hours). The combination of a sluggish or undersized rotor and non-active life style can lead to a watch that is not fully wound and running slow.
You cannot overwind an automatic watch, if it is in good working condition and properly serviced. There is a sliding coupling " clutch-like device" that allows the mainspring to slip inside the mainbarrel when this spring is wound up to a set point (below maximum spring winding). This slipping mechamism prevents overwinding and breaking of the mainspring. To work properly, this mechanism relies on a lubrication (containing molybdenum sulfide) that may not work properly in an old watch long out of service.
Winding a watch by hand is OK, but not really necessary if you have an available minute or two in your busy schedule. If your watch is relatively new, with 360 degree freedom of rotation on the rotor axis, simply hold your watch face up and 'swirl' it around at 2-3 rpm, the same motion with which you would swirl brandy in a snifter (or fruit juice if you are a Mormon or Moslem or simply a teetotaler). Do this for 1-2 minutes and the watch will have a couple of hours' worth of power reserve accumulated. Then wear it and the rest of the power reserve will build up fairly quickly. A Rolex, for example, accumulates about 3 minutes' running time for every turn of the rotor. Twenty turns of the rotor give you an hour's power reserve. 60 turns, not a very demanding effort, give you 3 hours.
The above is for a 360 degree rotor. For a 'bumper' such as used in non-Rolex automatics up to the '60s, gently shake the watch side to side for a couple of minutes. You should hear and feel the bumping if the orientation of the watch is right.