Zipping back to the start of the thread, reasonable accuracy regarding fuel is all that is required. For the last 13-14 years or so, I've only ever used kilos except for when you write up the trip paperwork where you enter the litres uplifted.
On the bug smashers I fly, the flight plan contains numbers to the nearest whole kilo so we use these numbers for planning. For ordering, most of us round. Some of us round up and some down. Either way, the over-run on the refuelling system will be about 60kgs. In flight, we'll round again and most of us round conservatively, or pessimistically if you like. It's always nice to know you have enough fuel. As for post flight, the exact number is used for fuel burn statistics. We'll also use the precise fuel density described in as many decimal points as supplied - apparently we pay less in "green tax" if we use the correct numbers. But it is important to consider that the aero engine manufacturer probably did their sums starting in pounds calculated from fuel flow units calibrated in gallons per hour. This was then converted it to kilos and used in an FMS system initially designed to work in pounds. The fuel density is normally "guessed" at 0.8 but it certainly varies between 0.794 to 0.804. The saying "Measured with a micrometer, marked with chalk and chopped with an axe" springs to mind.
But for navigation, I think most of us will enter our starting position as accurately as we possibly can. Small errors at the start of a flight build up to massive ones at the end.
As for conversions - we have a little "You Beaut" page in our FMS that converts pounds, gallons, kilos, litres etc. If it can find it. Failing that the Mk.I noddle or the electronic shoveler.