I try to let the immigrants do whatever they're already good at, and only keep one of each skill type unless its something I know I'll need more of (miners). That usually means I end up with tons of haulers standing around the well (Urist McGlassmaker doesn't usually have much actual work to do) but I ignore the ~40+ idlers for two reasons; I know that whenever I need to empty a newly dug out area of stone, it will happen quickly, and if I need to produce something for a moody dwarf there's always someone to take care of it, right away. (sometimes I have to put an end to a party, but that's no big deal) It also means it's easier to keep those non-rotting vermin corpses piled up in my trash-compactor room. In the end, I keep a food chain running round the clock, miners at the ready for expansion/expeditions, woodworkers and masons ready to follow up the miners' expansion work, and that's pretty much all I concentrate on until something fun happens. If nothing fun happens for a while and things slow down, I usually spend a little time creating some mechanism that doesn't come standard in all my forts, like a fountain or craft production area, or adamantine military equipment. This is usually about the time I find that I've designed something wrong, or I realize there is a way for goblins to actually get on top of my courtyard wall and shoot in, or some other calamity befalls and I start over.