There are no strong rules about comments, but I believe that it is good to comment public/internal/protected methods.
Sometimes I comment private methods when they are not very clear. Ideally code should be self-documented. For example if you have a method like
Item GetItemByTitle(string title)
then it is not required to write comments, because it's clear enough. But if a method could be unclear for other developers, please put your comments or rename/refactor the method event if it's private. Personally I prefer to read code, not comments :) If you have too many comments code becomes hard to read. My rule is to use comments only when it is required.
If on your project you have a convenience to document all methods including private methods, then follow this rule.