Through a certain series of circumstances, I got into an argument about tobacco recently. I was involved in what we're going to loosely term a writing collaboration, and the point is that someone wanted to add tobacco to a scene that was taking place in the 1300s. So naturally, I had a complete fit about it. Tobacco wasn't brought to Europe until after the famed voyage of Columbus in 1492, and you don't even have to be a history buff (like me) to know that. I got (metaphorically) bloodied in the ensuing argument that erupted, and it got me to thinking about disregarding history...and when it's okay to do that, for the sake of the story.
What Does It Matter?
I'm a big fan of historical accuracy, and I've been known to go into a rant about Disney movies that will almost shatter windows (many are inaccurate). So I'm always going to be the person who says everything should be historically accurate to the tiniest details. If tobacco didn't exist, then no your character can't smoke a pipe. You wouldn't paint over the Mona Lisa to put an iPhone in her hand, right?
But rewriting history is becoming popular in fiction these days, and there are times when it can be fun to see a story where the lines of reality are a little blurred. "Inglorious Basterds" won all kinds of acclaim, and the only true thing about that movie is that Hitler did exist.
Sometimes, history may need to be disregarded or otherwise tweaked in order to tell a certain story. The entire steampunk genre, for example. But sometimes, writers are just being plain old lazy. Don't disregard history because you don't feel like fact-checking. Don't gloss over or ignore or say it's all for the sake of the story when you could just as easily use real history to lend your story weight.
When you're disregarding history, you're walking a very fine storytelling line. If you're going to cross it, make sure you've got a darn good reason for doing that. Otherwise, you may end up in a knock-down, drag-out with the likes of me.