There is nothing wrong with a little common sense like Rat 5 suggests. Just make sure that you don't modify things to a degree where you are noticeably different from other F/O's as this can have unforeseen effects.
Don't do things that are supposed to be done in the cruise ( calculations, FMC entries etc) during the pre-flight phase, any spare time should be used to make sure all is in order. Eg there is no point in having created a waypoint for PNR in the FMC but failed to notice that the refueller hasn't arrived.
My thoughts.
Cheers.