So I've talked about research and its importance to crafting a vibrant story -- you don't want to make a faux paus like having a fictional doctor prescribe the wrong medication (unless that's part of your plot), for example. Similarly, you don't want to give knowledge to a 17th century natural philosopher that wasn't discovered until well into the 20th century. More important than this, though, is knowing which information to include and which to leave out.
In this manner, one rule and one rule only needs to be considered: What is important to your plot?
You can have books upon books of research on 17th century science or 19th century medicine (I'm almost certain such things exist), but if it's not germane to your story then there's little to no point in including it anywhere within your text. An example from my own writing -- one main character started his life on a farm. I show little to no aspects of farm life in the story, except in the very beginning of the story where he's discovering certain things about himself while doing the farm work. I'm not going to do a lot of research on the proper operation of a Renaissance-era family farm when the farming section of the book is perhaps fifteen to twenty pages out of more than 200. There's no point to communicating that much detail when it doesn't influence the plot.
Similarly, the steam-engine science and biogenetics of my current WiP are at or slightly above basic understanding. That's all the research I need to include in order to craft a vibrant world in the story. I could go into much more detail, and perhaps might now that I think about it, but one of the things I know I'll have to remember is not including details for their own sake. If it's relevant to the plot, then it's included. Otherwise it's left on the proverbial cutting room floor.
What about your own WiPs? What is the most interesting piece of research you've done that wasn't important enough to include in your story?