How often do you guys give interview without studying?

Lately, I've this strong feeling that I should be honest in an interview and present myself as I would appear everyday at work. What this means is I may not know some of the algorithms but have information about time complexity etc about it (Red-Black trees ahem). So, I just appear in the interview and most of the time I just do fine. Many times I'm hitting the optimum solution for problems/data structure even if I don't prepare for them. Sometimes I totally miss though as it is hard to come up with the solution if you don't know about it earlier.

How many of you think this is the right way to appear in an interview or may be conduct one ?