My understanding is that they only load as much fuel as they need to give themselves a reasonable margin of safety to their destination and then some... the issue is that they don't want to land with a heavy load of fuel for safety reasons -- the plane is easier to handle without the extra fuel and of course a crash is more dangerous with lots of fuel onboard. Unfortunately, the ideal person to answer this question isn't spending time on AB any more: Auntie Em. Her husband is a commercial pilot.

No. They carry usually only as much fuel as they need. Carrying more fuel than necessary costs money to carry the extra weight. There are exceptions - it they are flying between places where fuel is expensive and where it is cheap, they will overfuel at the cheap end. An example was the El Al freighter which crashed at Amsterdam: it was overfuelled because fuel is much more expensive in Israel then Amsterdam. Normal safety margins are enough fuel to (a) divert from the destination to a backup and (b) to loiter for 90 minutes.

A Plane only fills its tanks to a level that they can arrive safely at a destination with the weather conditions taken into account. Sometimes if conditions are very much tougher than expected a plane may need to take a fuel stop.This is not common but does occasionally happen. It is unusual for them to have a lot of fuel left on arrival at destination, if they have too much for some reason they will sometimes dump fuel for safety reasons.

That depends on the flight and the weather. Sometimes yes, sometimes no. A 747 or 777 going international, long haul with weather expected at the destination may have almost full or full tanks. A 737 or A320 on a domestic flight landing in good weather may have the minimum necessary for the journey. The "minimum" being defined as enough fuel to fly to the airport to which it is dispatched, plus fuel needed to fly to and land at the most distant alternate airport (where required for weather) plus enough fuel for 45 minutes at normal cruising fuel consumption after that. Also, the airlines sometimes DO chose to fill the aircrafts tanks with full (or very near) fuel even if it is not needed. This is called "tankering" it to the next destination. Tankering is usually done when the fuel is more expensive at the destination than where the plane departed, so the airline would rather take a weight penalty than pay the higher price. Firebrand said, "if they have too much for some reason they will sometimes dump fuel for safety reasons". Not so. Only in the event of an emergency will fuel be dumped and even then, not all airliners have the capacity to do so. FAA regulations on airline fuel, §121.639 - §121.646, can be found here- http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=5a8923527fb9b10aca70baf6adcfabd3&tpl=/ecfrbrowse/Title14/14cfr121_main_02.tpl

I am new to this, but I am an airline captain. I only have one thing to add, everyone so far has been right regarding fuel. Obviously enough to get to the destination, plus one or more alternate airports, and then some more in case a holding pattern is needed. If delays are expected, more is taken. Also, sometimes planes are filled full in the case where fuel would be too expensive to purchase at the first destination. Lastly, some elevations require weight restriction. A famous example of this was the nonstop JFK-Johannesburg route. When departing from JFK, a heavy full fueled plane has a large sea-level runway, whereas in Johannesburg, the elevation is considerably higher. When departing from Johannesburg for JFK, the flight cannot take off with full fuel, but instead stops for fuel after landing at a lower elevation runway.

So the answer is sometimes. A good example of a flight on which maximum fuel will be carried are the 747-400 operations from LA to Melbourne, Australia.