Hrm. I do think the proper research is important... but sometimes common words or phrases from a different time just wouldn't make sense to today's reader. Unless you're writing for the hardcore historian, I think in those cases it's all right.
Or maybe I'm just making excuses for myself-- I've been working on a medieval book for nearly five years, and when I went looking for medieval curses and cuss words, I took one look and said, "Forget it".

Just... *too* different.