You should never "add" comments - they are not additions. Comments are part of the code - you use them when you need them. Asking when you should add them is like asking when you should add functions or classes. Though thinking about it, I remember doing a program advice slot at university I worked for where one of the students came in with about 1000 lines of Pascal, with no functions. When I queried why he hadn't used functions, his response was "I'll add them later, once I've got it working."
