Personally I would migrate only questions that are obviously off-topic on Stackoverflow. If the question just begins: "I am making a game", edit it and remove the opener. Migrating is a good way to "save" a question, that can be useful on GD and would be probably closed on SO - since not many people know about GD there. And I am sure there are more game programmers on SO than on GD.
However the system has review, so if you feel that it should be moved, flag it (for moderator attention I guess) and comment on it. You can also bring this question to chat on GD where GD mods hang out often.
Maybe even start a question on meta whether a specific question should be migrated?