I'd do a little experimenting on your own with this to determine where you want it. I would think that lower is better, but if your dehumidifier fan is on 24/7 maybe you could find a happy medium where it's only kicking on for a few hours a day when the humidity calls for it.
The advantage of low humidity (other than preventing mold issues) is that you're going to be able to use evaporative (fans over the water surface) cooling more effectively than if your humidity was high..
For reference, the humidity in my house right now is 49% (running a humidifier for my sick boy, and no AC running to dehumidify). However, i have a small covered tank and don't lose much to evap at all.