What is the recommended Temperature setting for dual zone homes when not at home?
tl;dr Trial-and-error is your best bet. There's no standard answer. 

There's really no such thing as "working hard". Efficiency is mostly a matter of minimizing total system runtime needed to keep you comfortable during a given 24-hour period. 

Since every home and every HVAC system is different, you have a few considerations:


How quickly your HVAC system can recover X number of degrees
Whether the humidity that accumulates during the day is a concern


A couple tips:


Discontinue cooling up to an hour before you leave, but keep the fan running at least periodically. Your home won't probably start warming up enough to be uncomfortable. 
Gradually increase the "away" temperatures until you find that recovery isn't happening in a timely manner or humidity becomes a problem. 


Your climate and mine are similar, and I find that I don't need to maintain cooling at all during the day for all but a few days out of the year. Again, keep the fan on a periodic cycle to prevent stagnation and hotspots due to solar gain.