When to use callbacks?

I don't quite understand the use of callbacks in node.js. I understand that if you have something like

result = db.execute(query);
doSomething(result);

you should make doSomething a callback because doSomething would get executed before the result is ready. This makes sense because the db operation can be expensive.

Now let's say I have something like

result = calculate(x,y)
doSomething(result)

where calculate is not expensive (i.e. no reading from database or I/O), should I still be using a callback? How can I tell if my function would complete before or after the next line would get executed?

Thanks