When to start leveling my professions

[Gromgo]

This is the way I would prioritize, ( I don't mean gathering professions are not useful, just they don't have to be on your main team):
Lowest priority: skinning and herbalism, can be skilled to 350 in one day at no cost.
Next: mining, harder to skill, but still at no cost.
Next: fishing, yes, mats for alchemy, cooking and dailies, ranks above normal gathering professions
Next: engineering, hard to skill, expensive, requires mats from all gathering professions. I love engineering on my paladin tank for pulling, but I treat it as a gathering profession
Next: blacksmithing, not really that useful, unless you have many plate wearers
Next: leatherworking, depends how many wear leather or mail, can produce spell power and attack power gear
Nice to have: inscription, glyphs are expensive but easy and cheap to learn (at least the discoverable ones)
Nice to have: alchemy, create your own potions, transmute is nice, but I get most of my gems from the pvp vendor lately
Nice to have: jewelcrafting, create rings, necklaces and gems at no extra cost
Important: tailoring, create your own cloth armor, most classes wear cloth, all classes can wear cloth
Very important: enchanting, period.

Keep in mind: mats of the current expansion are cheap compared to mats found in vanilla or previous expansions. Craft-able gear is worth less later in an expansion than at the beginning. Mats will almost always be more expansive than the finished product.

Further consider: where do recipes drop? If, like in WotLk, jewelcrafting recipes drop in instances, your main team that does instances should include a jewelcrafter. This pretty much dictates what professions are in what team.

When to skill professions: as late as possible. The greater the reputation, the cheaper to buy skills (I guess, I have never actually compared chars with different reputations). The higher the char's level, the easier to farm mats and recipes.