You do not want to take off downhill if it can be avoided.
In the first place just about all airliners have a type limitation of 2% runway gradient. That is two feet of incline per hundred feet of runway length, or "rise over run" to calculate it. Very few airports in the devleoped world have slopes that exceed this. Aspen Colorado does, and airlines have to get a waiver by "demonstating an equivalent level of safety" in order to operate there. What it really means is that they are severely weight-restricted there. I believe that it is customary to take off downhill because of rising terrain at the uphill end.
Remember that takeoff performance is not all about getting off the ground. It is largely based on safety with an engine failure. Taking off downhill seriously increases stopping distance in the event of a reject. So down slope decreases V1 and we would, if anything, prefer to take off up hill. We just don't want to go wallowing out over rising terrain if we lose an engine.
edit: I answered the above from an airline, or multiengine point of view. In singles, and particularly STOL ops, taking off down a hill would probably be better in most cases. Most extreme runway I ever used regularly was about 1200' long and sloped a full 200' in that distance. I landed up the hill and took off down the hill. If there was any wind at all I did not go in there. The first plane to use the strip crashed on takeoff. They took a Cat back and knocked down a couple of humps and I was the second plane to use it. (Cessna 206 with a STOL kit) The good news was, it was not necessary that you be able to climb because the canyon sloped away sharply from the bottom end of the strip. I forgot to mention that it was roughly seven thousand feet above sea level.
[Edited 2005-11-20 18:02:43]
[Edited 2005-11-20 18:05:59]
Happiness is not seeing another trite Ste. Maarten photo all week long.