You do have the option of saving the company money. If it is considered a worthwhile tanking sector, you must consider the maximum fuel you can land with for landing purposes and also ground icing. For a positive sector, yoou should take up to the maximum. It may not be much saving, but you are one flight of many that day that could contribute over a year to a significant saving. It takes a reasonable amount of pragmatism and common sense, but they have given you a figure for guidance. They think it's worth it. They pay your salary. Your job is to do as you are requested by the company, following their guidelines, as long as it's safe and what your flight manager thinks is 'sensible'. If you disagree, you should not just disobey your guidance. You should go in and discuss it with the flight manager to see what you should do. You may well find that if you are not prepared to follow company guidance, you will be invited to leave! They pay you, it is their ball. You do it their way, not yours.
You must always remember far more knowledgeable people than you in the company have worked this out. You should not just ignore guidance- if you disagree, go discuss it with them. There are a lot of other factors involved. Local contracts for a minimum amount of fuel over a period, other fuelling costs, fuel shortages etc. They are not paying you for you to decide you don't want to do it their way!