>>>Is ist true, especially at carriers in the US, the commander has to specify/report why he/she ordered fuel above MINTOF?
Speaking as a dispatcher operating under US FAR 121 Domestic/Flag rules (NOT what a dispatcher is on that side of the pond), the above is usally a company requirement.
I endeavor to consider the likely possibilities when I plan fuel loads, and personally, I have zero problem should a PIC want to discuss adding more. Using 737 values, if s/he wants another 1,000-2,000 lbs, I usually don't have a problem with that, but will ask why they feel they need it. Not to second guess their rationale, but to crosscheck myself, i.e. have they considered something that I missed in my assessment/planning, and more importantly, is it something that I need to apply to -other- flights.
On rare occasion, I'll get a PIC who wants to add 4,000+ lbs. and most of these requests seem to be rooted in the "I flew this leg last week and....." when the weather today is completely the opposite of the way it had been the week before. After discussing -today's- weather, we usually agree to something less of an increase. On the even rarer instances where the PIC insists of 4,000+ more and there is no weather/operational need for it, I'll usually conference in a chief pilot on the phone. He'll get his fuel for the immediate flight at hand, but his CP will discuss it with him later. Like I said, that's a rare instance. Most crews and dispatchers here converse and coordinate fuel loads without too much of a problem.