I try to adapt to the environment.
When developing for an Windows application, I use the Windows Vista User Experience Guidelines but when I'm developing an web application I use the appropriate guidelines, because I develop Dutch websites I use the "Drempelvrij" guidelines which are based on the Web Content Accessibility Guidelines (WCAG 1.0) by the World Wide Web Consortium (W3C).
The reason I do this is to reduce the learning curve of a new user.