It depends on the company. I used to work for a cargo carrier that was always second guessing the crews...especially when they wanted to get that last few hundred pounds on. As has already been stated, the parking brake is the final arbiter.

Also already stated, don't ask why. Just tell them what's acceptable and what it's going to take for the airplane to leave.

Two caveats: first, be calm and matter of fact; second, be damn sure your right. If you follow those two principles, eventually the word will get around and you won't have these conflicts. The powers that wannabe will decide to leave you alone because you're not negotiating AND because you're generally right. This works for other parts of the operation, too. Scheduling in particular.

A side benefit of being right: if you ARE called into the chief pilot's office, the discussion will be a lot less painful.