Once you have mixed the gasoline you have created a chemical mixture that changes the boiling point of the gasoline. In unmixed amounts the boiling point of gasoline is around 85C, whereas water is 100C, so even in a mixture you will lose gasoline out of the mix, but you will also lose water. I heat my oil probably more than anyone here does, and I notice some loss of gasoline, but also notice a huge difference in the amount of water after I heat. I'm currently heating with 5500W. My indicator that I'm losing too much gasoline is you start to get an opaque white steam vs the darker more translucent steam given off by water. I will cut the heat at that point, as I don't yet have it set up on a thermostat.
But I would definitely suggest heating, because it is possible for water to hold dirt in emulsion just as oil holds water in emulsion. Keep the temp under 180F and you won't lose gas to boiling. Heating the oil helps to mechanically, and chemically remove water, if that makes any sense worded that way. Better put is it will further lower the viscosity of the oil which allows emulsified oil to centrifuge out and the the water that is chemically bonded will evaporate out, but as you suggest, some of the gasoline will also. Necessary evil as I usually put it.
