Its all in the POH that you'll get to read before you fly hopefully.
The Thielert Engined C172 is approved to use diesel (DIN EN 590), Jet A, Jet A-1 and various other fuels in any mixture ratio.
Each different fuel has its own minimum inflight temperature, if you have a certain ammount of diesel mixed with jet A-1 (ratio in POH) then you treat the fuel as though it is diesel. I don't have access to the POH today, but if you use Jet fuel then the min temp. is very low.
Jet fuel is our primary fuel - its cheaper, no temp issues, quality control should be better, has the anti fugals to stop the black death and is very slighly less smelly and slimy - you'll still end up smelling like a pole cat though. Diesel is a useful "get you home fuel", its available everywhere (off airfields though), and does not require another STC/AN98 compliance/public transport limitations etc.
Another thing to bear in mind is the fuel in the tanks is heated by the returned fuel from the high pressure pump system. The engine draws down a huge amount of fuel, pressurises it to several gazillion psi (heating it up), uses a tiny fraction and returns the rest to the tanks. They are noticeabley warmer after flying. Each tank has a digital temperature gage with a colour coded bar graph on the combined engine instrument gauge.
I've never had a fuel temp issue, I think you'd die from hypothermia first. The heater is rubbish - it uses a heat exchanger from the coolant instead of the exhaust muffler. You won't die of CO poisoning any more, the cold will get you instead.