I think the wiper arm posts are splined so they can't slip. Don't reposition the wipers if they are travelling through their normal range of motion.
If the wipers are consistently stopping higher up on the windshield, the problem may be the drive crank at the wiper motor assembly is slipping. This is located under the driver's side ventilation grill just in front of the windshield. You'll have to pull off the driver's wiper to get the cover of. The heater blower motor has to be removed to get the wiper motor and linkage assemblies out but you may have enough access to check the tighness of the nut holding the drive crank through the ventilation cover.
This assembly has a park switch inside so the motor stops only in one place. The drive crank should be positioned on the assembly so that remaining passenger side wiper (you left that one on, right?) is in the right place when the drive crank nut is tightened again. When looking at it through the cover I think the crank should be to the left and slighly down.
Your bentley manual has all the specifics.
Steve