I've written my answer in a blogpost. thought it would be to long as a comment. Hope you permit this?http://rrsstudy.blogspot.com/2009/05/rules-answer-for-tillerman.html
The rules pretty much never require you to sail your proper course. They do, however, put limits on the course you can sail that are based on your hypothetical proper course.1. You can't sail above your proper course when you have gained a leeward overlap from astern within two boatlengths of another boat (unless the overlapbegins while the windward boat is required by rule 13 to keep clear, i.e., they tacked into the overlap).2. Broadly speaking, the markroom you are allowed under rule 18 only includes your proper course while at the mark.3. You are not allowed to interfere with boats on another leg or boats that are completing penalty turns unless you are sailing your proper course.
You guys are too smart to fall for my trick question. Yes, you are both correct. Contrary to the belief of many racing sailors, the Racing Rules of Sailing do not require you to sail your proper course in any situation.Well done! You will share the first prize of a lifetime free subscription to... Proper Course.
Some people struggle with when and what proper course is.This article on LTW might helphttp://rrsstudy.blogspot.com/2009/02/proper-course.html