John, I don't think it matters too much in your situation.

If the upstairs is not used it doesn't hurt any to shut it off or keep it set really high. Your HVAC system is for your comfort, really nothing more than that in most situations.

The AC system (any AC system) is designed to drop temperature 15 to 20 degrees from return air temperature. The term: 'working a system hard' is misconstrued.

If the return air is 87, when the system is run the supply air should be in a range of 72-67 degree air. As the return air drops so will the supply air.

If the return air is 75, when the system is run the supply air should be in a range of 60-55 degree air.

The AC system is under the same work load regardless of the setting of the thermostat. It's going to deliver 15-20 degree air difference of the return air regardless (provided it is operating properly).

Problems are more likely to occur if the system is not working properly. Excessive operating temperatures can occur regardless of the return air temperature or how high the setting is on the thermostat.

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Efficiency in terms of monetary savings is an entirely different subject. If you like it warmer in your house a more efficient system in terms of energy usage will cost you more to install. Because you like it warmer, it's counter productive. To increase your efficiency you merely shut the system off. However, shutting a AC system off doesn't make the system energy efficient in the true sense of the term.

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If this situation were flipped and you were living in the upstairs portion of this home, things would not work so well if you decided to shut the down stairs system off and try to maintain the upstairs temperature throughout the entire summer. For the simple reason that heat rises / cool air falls.