Desp its not about fuel its about money!!
The world is not about to run out of oil anytime soon, Airlines on the other hand are about to run out of money if oil stays where it is price wise, so the effort is to save money and burning less of the stuff will do that.
If the price was the same all over the world no one would tanker would they, if the price is? 40% dearer in Slovina then the cost of the additional fuel burn in carrying extra fuel is offset by the lower cost of the fuel uplifted and tankered.
My local fuel station is not the cheapest, but if i am filling the car up it is cheaper for me to drive the additional 10 miles round trip to which is paid for by the 7-9p a litre saving on a full tank.
We are all agreed that we burn extra fuel to carry extra weight so it is only ever a question of how much is enough. Issues such as the accuracy of fuel gauges and the cost of a divert are IMHO a red herring.
Diverts
I fly around 650 sector a year and have diverted just once in the last three years, in the last 8-10 years it will be no more than a dozen times, i normally operate into major airports in a CAT3 aircraft, if operating into smaller fields with NPA and aircraft that are CAT1 only then yes the number of diverts will increase, but by and large these will be turboprop type ops with much less fuel burn in the first place! my point here is that the cost of carrying an extra 500kg for mum over the last 3 years and nearly 2000 sectors would have been far more than the cost of a divert. I could also add that even if i had an extra 5000kg on i still would have had to divert because it was 300m and the airfield was only CAT1 anyway.
Gauges
Its often the case that on the 737 classic the centre tank will magic some fuel from somewhere so unless i have put some in there i ignore it, but i will try and burn it off in the CRZ, who cares about 20kg? does it matter if you land with 2500 or 2520? any slope on the ramp can change that, let alone the fuel temp so if your worried about 20kgs what are you flying? a C152?
At the end of the day it is the Captains call after discussion with his/her F/o, but you are being paid to make SAFE and then COMMERCIAL judgements otherwise you might as well fill her up