I found out via a under carbonated batch of porter that you need to use the highest temp the beer reaches after fermentation is complete. The reason is this, The temperature is used to adjust your priming sugar amount to compensate for the amount of Co2 retained in the beer. Again, this compensation is temperature based.
When beer gets warm Co2 comes out of solution. If fermentation is complete then there is no more Co2 being produced, so even if the beer gets cooler again it will maintain the same Vol of Co2 as when it was at the warmer temperature. So going with the lower temp will result in low/inaccurate Vol's of Co2.