General run of thumb, run 'er wide open for all tools. Except... for smaller sanders (ETS 125, DTS 400, RTS 400, etc.) run at about half speed. You can actually run the sander and slowly turn down the suction until you feel the sander "break free" and start to float on the material.
One other consideration is to dial down the CT for more power to the tool in instances such as cutting thick hardwoods with your TS or hogging off material with a big router and bit.