It’s generally not a good idea:
Expectations
Users, especially power users, expect to be able to right-click on elements in desktop applications in order to get a menu of element-specific actions. This expectation does not exist for web applications – indeed, the expectation is that right-clicking in a web page will give you the standard web-page menu where you can print, open links in new windows, view source, etc.
Reliability
Because the ability to override built-in menus has been so abused in the past (mostly by naive programmers attempting to disable saving images), many browsers prohibit it or make it difficult for client code to override in a reliable fashion.
The Exception
If you are creating a web app that closely simulates the behavior of an existing and well-known desktop app, it may be appropriate to put the effort in required to implement sensible right-click menus. However, you should then also follow the usual recommendation for these menus in desktop apps: use them to provide quick access to context-specific operations, but also provide another means of accessing the same features.