I just had to straighten 2mm wire when I was making my pentacle as the branches of the star needed to be absolutely straight, and I used my hammer on steal plate.
To check if it was straight enough, I just rolled it on the table (a bit like with a rolling pin) with one hand at either end of the piece of wire. It was easier to spot where it needed straightening (anything sticking up).
Not sure if it helps, and sorry if that's an obvious tip and everybody uses that technique...