When is it a good idea to clean a solution?

As开发者_如何学编程 the title says - what are your guidelines for when you should run a "Clean solution" on your visual studio projects?

Whenever I am having problems I can locate in 30 seconds or just before I build a release version (switch to release mode first, then clean).

If debugging (between breakpoints) becomes very very slow... Clean and Rebuild. If post build operations suddenly fails. and Whenever you feel like you have an extra few minutes... never hurts...

When you updated some assemblies your solution depends on, it is sometimes necessary to clean the solution, but I have to say that VS2008 does a very good job at detecting changes to the references, so it's not really needed anymore (previous versions of VS sometimes had problems with updated references).