I don't like the threaded rod technique as it presupposes the two sides are equally bendable, which they rarely are. Put a cotton thread round the dropout slots and head tube; the distance between the thread and the seat tube has to be the same on each side, or one side has been squeezed more than the other.
I would put the frame on some cushions, ensuring the main triangle is supporting it, and push each side in independently. Keep checking the side-to-side alignment and the spacing.
If you're running a hub gear, which can malfunction due to the slightest bend in it (causing internal components to bind), you also need to return the dropouts to parallel afterwards, and this is very hard to do with cast or forged dropouts. For anything else, it doesn't matter too much.