Every flight my dispatcher figures the fuel and I either sign off on it or change it. These days our dispatchers are trying to keep the fuel load as low as possible and still give the crew options if they need to deviate or the flight doesn't go as planned. I find they often put about 1000 pounds extra on my flights. I usually find their calculations adequate. If I want more fuel, it is usually because I want to fly fast and get home ASAP.

I find it amusing when some captains I know get all excited because they want an extra 500 pounds. I've seen them call out the fuel truck and demand the fuel. It is their right but do they realize that 500 pounds is only 5 minutes of cruise?

The time to argue about more fuel is when you have info that the dispatcher may not be aware of or is not thinking of. For example, there may be a huge line up for take off, or, you may be using a runway much farther away than normal. In these cases you need more taxi fuel. You are in a better position to judge than the dispatcher. Also, you just landed and you know the head wind is much stronger than your flight plan forecasts for the flight back home. Or you know the file altitude is rough so you will need to fly lower.

I have never been denied extra fuel. Like someone else said, don't get upset about an argument you are going to win.