I guess I'm the practical scientist type.
I know that SI requires me to use the SI names and constants like N, Nm, kg, m/s2 and so forth, and I understand the relations between them. I can (or rather: could) do differential and integral calculations and such so that I could tell you the speed of a falling object, and the distance it has travelled, after n seconds of letting go, even considering air resistance, and the other way around.
I also know that in real life gravity is constant, and by using weight and mass interchangeably and measuring it in kg (or lbs if it's a US built airplane) the calculations are greatly reduced in complexity and still very accurate (unless you intend to fly to the moon and back). So for all practical purposes I do the W&B (or M&B) sheet in whatever units the airplane manufacturer specifies, and I interchangeably put USG or liters of fuel in the tanks. I do make a point of writing the units used after the number though. so I don't get confused later on.
And it also allows me to claim that I weighted 360 kilos yesterday, and only 45 kilos a few moments later. Try that with the Atkins diet! (I had my third aerobatics lesson yesterday.)
And on a related note: I'm still looking for an E-6B which has the density of Jet-A/Diesel on it, in addition to 100LL. Anybody knows where to get one?