I'm new here, and I realise I may be re-igniting :-) an old debate, but one thing I have read in several threads is that at some temperature below 0C, the butane in a propane/butane mix stops boiling, or is too cold to evaporate, and so only the propane is used when the canister is in an upright position.
This is not the case. In a propane/butane mix, the butane and the propane each have a vapour pressure which depends on the ratio (the molar fraction) of each in the liquid mixture and the temperature. The vapour pressure of butane and propane are both a smooth function of temperature, there are no sudden jumps at the atmospheric boiling point. Furthermore, the vapour pressure of each are both affected very similarly by temperature changes, and the RATIO of the vapour pressure of each changes very little as the temperature changes. As a result, the ratio of the two gasses in the gaseous phase (which is what comes out when the valve is opened) changes very little at different temperatures.
So for example, for a NEW canister containing 30/70 propane/n-butane in the liquid mixture, at -20C the gas mixture is 70% propane, whereas at +10C the gas mixture is 65% propane - not a significant difference at all. This percentage does change as the canister is used (because the molar fraction in the liquid changes), and while temperature obviously affects the total pressure, it does not significantly affect the ratio of each gas used.
To illustrate the point, I have calculated the boiling point of various mixtures as the canister is used: regular 30/70 propane/n-butane, 25/25/50 propane/i-butane/n-butane and 20/80 propane/i-butane for those lucky enough to be able to get it.
To be clear: the boiling point is the temperature at which the total vapour pressure equals atmospheric pressure. Below the boiling point air will go into the canister when you open the valve, above the boiling point and the gas _mixture_ comes out. All three gas mixtures are much the same when the canister is new, but the dfferences becomes clear as the canister is used. I reckon the canister needs to be at least 5C above the boiling point for there to be enough pressure to run a stove.