As I'm building up to a mature fortress, I usually pick jobs in areas I need if the migrants show up with things I don't need. Although I have no evidence there's any skill crossover, I will usually activate (for example) weapon smith and armor smith labors for that novice metalcrafter that arrives. If they show up with something useful (brewer, carpenter) then I'll activate the related labors as well so they can make their own raw materials (so no dwarf ever has to wait for another dwarf to do their jobs). For example:
Soap maker and lye makers, I activate wood burning, lye making, cooking or pressing (for the tallow/oil), and soap making. Sometimes I'll add these labors onto existing doctor-types so they can make their own soap when they're not fixing the injured.
Anyone with food prep related I'll activate growing, milling, plant processing, and cooking
Anyone with animal processing I'll activate butchery, tanning, and leatherworking
Anyone with metal related I'll activate furnace operator, metalcrafter, and either weaponsmith or armorsmith (sometimes both), although sometimes I'll leave most of the forge dwarves as furnace operators since I generally want lots of dwarves smelting ore and melting down goblinite, but only a couple highly specialized dwarves forging high-quality weapons and armor.
Anyone with stone-related skills I'll activate mason, engraving, and architecture (so the masons don't have to wait for the one architect to design their buildings).
And so forth. The only workers I leave dedicated to a single task are typically miners (I turn off everything for them except mining, burial, and clean), woodcutters, hunters, and fisherdwarves; miners, because I want them to be responsive when I need something dug out, and the rest because I don't want them wasting more time walking back and forth across the map to do both internal jobs and external jobs. They should only return to base to eat and sleep.
I also sometimes leave dedicated stone crafters and carpenters, since they're usually spending their whole time cranking out stone crafts, rock pots, wooden bins, and beds. Bone crafters as well for the bone bolts and totems (to clear out all of those otherwise-useless skulls).