Well, I myself try to keep things on the fantastic side of the line, but I'm not above throwing in a little realism every now and then. One of my players is a chemist with a minor in engineering, and he happens to like figuring out equations (so every now and then I ask him to crunch some numbers, since I do have some physics lessons under my belt). When that happens, it usually results in something so stupid its funny. But when it gets in the way of the rules/entertainment I let everyone know the textbooks are being put away for a bit.
For example, I've let the afore mentioned player use real-world physics as a means of finding clues in certain murder scenarios. He's very good at figuring these things out. It confuses a few of the others, though most of us are able to help him solve equations. Hell, there's been a time when we put his engineering minor to good use through creative trapmaking checks.
Yes, I am aware Catgirls are an endangered species.