Probably none.
Reading your post and thinking that this might be your first project, I can't help but think that you might not be talking about a twisted rudder but maybe a less than perfectly straight trailing edge of your rudder.
With today's pre punched kits, it seems almost difficult to make that rudder twisted in any way.
If indeed you are describing a "curvy" trailing edge it is not a big concern but merely a matter of aesthetics and to some a matter of pride.
Riveting a twisted control surface was much more common before the advent of pre punched kits.
If your rudder is twisted you need to have someone take a look at it.