What's the most conservative? That is your answer.
Just like an airliner will use a 'flex' temperature up to 69 degrees to reduce wear etc.
Personally I'd be using the highest forecast temperature on the TAF for the day which is how I planned my payloads in the regional days. If you can go max weight at 40 degrees for the runway Id be using that for my P charts.
When I was in GA I had a booklet that I made that had all my regular airfields with P charts based on 2000 feet/40 degrees and nil wind. I had a similar system for CPL training to make planning easier and give me more time in preflight prep to worry about how I was going to fly the route, fuel etc etc.