G'day I,
As an FO I tried to make a logical fuel decision but I was continually over ruled by whoever the captain was so I decided to just order the flight planned fuel, which couldn't be argued as it was company policy, and let the captain make his decision and avoid the drama and some of them were very dramatic.
A tip I was given was to never make any extra fuel decision an even 1000 but to make it a semi random figure in order to make the boss think that you had actually thought about it and calculated a sensible number.
Once I had my command things were much easier and I was never over ruled.
Cheers,
BH.