I currently have all the professions including gathering professions on all my characters of max level 85-84. There is one profession that you want to have at the start of any expansion and at the end, and that is going to be skinning.
The reason is that gathering is by far the most money per-hour at the start of any given new expansion. At the end, you want to re-level it so you’ll have it and be ready for the next. The best profession to replace skinning for is Alchemy.
Out of all the professions, it’s the cheapest to level and quickest. I would recommend setting aside an Alchemy Leveling Kit and try to sell it at some point at the start of an expansion. But be sure to do your research first.
If for whatever reason your unable to find a buyer, you can also use it to drop skinning and level Alchemy in its place once the overall prices have settled. This generally happens 3-4 months after the expansion has been out.
Professions are updated at least once every two weeks or so. When ever I remember to do so. Also I only keep track of professions on my two main servers I play on, Ravencrest and Drenden.
UPDATE: Due to quitting World of Warcraft, this page is now out of date. – 12/01/2012