Then provide a stable power source and don't think about temperatures (throttling will jump in if load peaks make it necessary). Apart from that it might help with longevity if temperatures are lower (disclaimer: I'm no hardware guy) and in case you have an instable power source improving heat dissipation is always counterproductive since then throttling isn't that aggressive and since voltage drops occur under high loads chances increase that CPU gets clocked higher, is driven with a higher VDD_CPUX core voltage and then the critical supplied DC-IN voltage falls below a certain treshold.
This is what seems to be hard to understand. The relationship between higher VDD_CPUX voltage (the SoC is fed with through a programmable voltage generator switching from eg. 1.1V to 1.3V which increases consumption) and dropping DC-IN voltage (eg. from 4.6V down to 4.3V which might already insufficient). The only relationship with temperatures is the aforementioned one: Improve heat dissipation only if you can provide a stable power source with stabilised 5V.
On Banana Pi (A20/AXP209) it's easy to measure since drivers are available for the PMU: http://forum.lemaker.org/forum.php?mod=viewthread&tid=8312&extra=page%3D1 and the difference between a crappy USB cable with AWG 26 or 28 rating and good short one with only AWG 20 can be seen here: http://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/
H3 has no PMU so you would need a multimeter to measure. And unfortunately the FriendlyARM folks chose the crappy Micro USB connector to power the board. In case you run into stability issues think about providing power through the GPIO header (pin 4: 5V, pin 6: GND)