Question: In general, do people think that an opening band should be one that plays music similar to the main band, or is it better to have an opening band that provides some real contrast to the main band?
I don't have a solid opinion on that, and I can see good arguments going either way. Similar music might be a safer choice, in that it has a better chance of appealing more to the core audience of the main band. But then they might not be so hungry for the main act to begin. If it's a very different kind of music, the contrast can help to offset or highlight the distinctive style of the main band, and the diversity of musical styles could keep some fans more engaged with what's going on. All that being said, it's also true that Jack White's musical palette is pretty broad, so he can easily provide his own diversity and contrast; and it's hard to imagine any other band being all that similar to him anyway.
Oh, and bands that should open for Jack White: Maybe Foo Fighters or Black Keys. Or, heck - maybe Radiohead or the Rolling Stones.
(Just to clarify, that remark was not serious, although I do like some of those bands. It was just a little joke about how much I would like to see Jack White headlining over every other band out there.)