I definitely only take migrant skills as suggestions. If they did like making lye, I'd probably stick them in woodburning, and add in a few soapmaking training things for the first one, then if more lye/soapmakers turned up, get them to do more wood burning and a bit of military.
Any cheesemakers who turn up, I teach 'em to milk. Any milkers who turn up, I teach 'em to make cheese. Nothing more annoying than a dwarf who milks heaps of milk and leaves it to rot because he doesn't have a friend who will cheesemake, and it's not like there's anything useful to do with milk if you don't cheese it. Same goes for shearers and spinners, for that matter.
Make shields as well as wheelbarrows. They stack really well (just stick some wooden-shield-only bins near your Depot) they're no good for elves, but humans and dwarf merchants will pay top dollar for good ones, and as an extra added bonus, your carpenters get REALLY good at carpenting (I think that's not a word) and you can then use them to make beds, tables and chairs which make happy thoughts when they're not pumping out hundreds of shields a season. Wheelbarrows suck as each one has to be carried to the Depot individually: Shields are great for putting in bins and get carried all at once, and they can sell for incredible amounts of money.
I'm told the same can be said for crutches, but I make sure all my crutches are made of heavy metals, so that they can be used as bludgeoning weapons if and when necessary. If there is even one wooden crutch in the fort, the medic will assign it and only it to the limping footless soldier.... But in theory those two would be bin-able and easy to get to the merchants on time.