I level gathering as I level but as for other trade skills, I wait for 60 or 70 depending on the tradeskill.

for my 5 shamans the tradeskills i have set up are as follows

1: Alchemy / Mining
2: Alchemy / Skinning
3: Alchemy / Herbing
4: Alchemy / Enchanting
5: Alchemy / Jewelcrafting

I picked up alchemy on all chracters as they are going to be 58+ and I refuse to waste a 68+ character as a non alchemist that can easily turn primal earth into water. dirt into gold .

Mining, skinning, herbing as gathering is very good money.
Enchanting for the obvious DE in instances, and Jewelcrafting to make sure I can turn down my ores into gems. Or cut gems for my alts.

Of course what trade skills a person takes is always up to the person, but this is what I like doing.