Well, like I said, depends on the fiction itself. If it's just in the milieu, then I want it to be well-researched, and it can be all right--and I am, after all, a die-hard Sherlock Holmes fan, which was contemporary fiction when it was written and is now historical. But if you stick in Abraham Lincoln (unless it's Real Holographic Simulated Evil Lincoln) or somebody it will begin to annoy me. (There are different degrees of tolerance for sci-fi time-travel stories, which I can get impatient with quite fast. I think it depends on how "possible" the authors try to insist it was ...)

I can handle something like the American Girl stories, that just have a person in those times, but once you cross into them taking part in actual historical stuff it gets obnoxious.