Our MEL contains the following statement:
"The decision of the Commander of the flight to have allowable inoperative items corrected prior to flight will take precedence over the provisions contained in the MEL. The Commander may exceptionally request requirements above the minimum listed, if in his judgement such added equipment is essential to the safety of a particular flight under the special conditions prevailing at the time"
I am quite sure, in my company, that if I raised a sensible objection to dispatching with an allowable defect that in my judgement could not safely be carried due to prevailing circumstances that the pilot management would back me up. We have a pilot manager (they take it in turns) who can be contacted 24/7 for any reason including just this sort of eventuality and I have always received good advice. The normal attitude is "do what you think is sensible and we will back you up".
That said there is of course commercial pressure to get the job done but as said above it is part and parcel of the job. Being a commercial pilot doesn't just mean you get paid to fly, you have to operate and make decisions in a commercial manner, if you can't you are in the wrong job.
As for fuel I think most guys load a sensible amount and are trusted to do so, its 100% down to the Captain and no questions are asked although I think if you routinely loaded tonnes extra then someone would have a word.