1. If it's a historical thing based on actual people, I'd prefer to be as accurate as possible. It's hard to have talent + looks for a specific role, but when it works, it works.
2. If it's based on source material, I'd prefer to not change the artist's vision.
Both have their own fan base (history and literature/comic nerds) that built the idea of the thing being made to begin with, so I think in those scenarios, it's best to stick with who ever works on the page.
What really pisses me off, is Tom Cruise playing a frenchie.