I'm pretty much agreeing with pdr all around, here. But just to add a thought...
I think a lot depends on why you're changing or inserting something.
One could write an alternate history where the whole point is changing one thing and then speculating logically from that point onward. What if... Roosevelt had been assassinated in 1942? Or Germany had developed the atomic bomb first? In that case, even nit-picking readers will accept your changing that one thing, if you then speculate as realistically as possible afterwards. On a lighter note, I suppose one could say the "real people as detective" genre is like that: what if Attila the Hun decided to solve a murder mystery?
Or one could write a book about a famous person, with the goal of telling his or her life in a more vivid way than could be done from the information that survives. In that case, the point is to be faithful to your vision of the historic person, even while making up whole scenes entirely, because the goal is to evoke the person for the reader.
Or one could write a book set in the past, the way books are set in the present, entirely making up your main characters or towns or businesses and inserting them into the real world, yet still having them seem realistic and believable. Though all the characters may be totally fictional in a modern romance, that doesn't mean the author can casually toss in other made-up things, like headlines about England bombing Canada, without readers going huh?
I think readers have a sense of what's okay to make up, and what seems like just a mistake. Obviously it'll vary from reader to reader, but I think it's well worth figuring out why a novel will be better if historic information is changed or rejected, other than just for the convenience of the plot, because readers will notice and wonder--and the ones who notice the most will be the readers who are already most passionate about the subject and therefore, potentially, your book.