Pandora, would it be possible for you to approach an instructor and tell him/her that you've become unsure of the company standard as you've flown with people (do NOT mention any names at this point) that order fuel in a way you consider contrary to company rules, and would the instructor please clarify the company rules again to you?
That way, you've demonstrated to the training dept. that there is at least one person operating in a non-standard manner, you are very keen to do the right thing but because of your position as FO are currently unable to enforce it. The training dept. can and should.
IMO it's a pretty serious offence to take less fuel than calculated, unless there are very good ways of making sure that you can make it up; in offshore heli ops it's often the choice between payload, performance (or lack of) and route fuel, but we can easily make an extra fuel stop if and when required.
But as soon as the fuel runs out offshore, as it does occasionally, you'll see very, very conservative fuel planning..