Remember the golden rule of migration:
Migration means “we want it”, not “let's shove it to them”.
In other words, you should migrate a question if think of the target site as “us” and you'd like the question on “your” site. Don't migrate a question if you think of the source site as “us” and of the target site as “them”.
Jeff's first rule of migration is a corollary:
Don't migrate crud.
Stack Overflow questions have often attracted answers that the more expert communities don't like so much. The target voting base is much smaller and often doesn't have the manpower to regulate the votes to better rate the questions (it would help if votes were reset upon migration.
If it's early in a site's beta, migrated questions can be disruptive because the target site hasn't fully decided on its limits. You don't want to migrate a question to a beta site only to have it declared off-topic after all.
If the site is already mature, there may well be a better-worded, better-answered duplicate on the target site already.
In practice, what that means is: do not mass-migrate questions to new sites. Do not migrate questions if you merely think “they” would be interested.
Ok, I've been repeatedly telling you not to migrate. But sometimes migrating a question can be a good thing. If you consider yourself a member of the target site's community, and you consider the question an especially valuable one for your site, and it's borderline or off-topic on SO, then flag it for migration (tell the moderator that you're to be trusted on your target site — being a moderator there would do it).
For new questions, you can lower the bar: migrate them if you think they'll get better answers on your site.