How wide can one extend a typeface?

I have always been looking for some rules of typeface usage and have still not managed to find anything.

For example how wide can one stretch a typeface before it becomes ugly?
As a guideline for myself I try to keep the horizontal and vertical bars optically similar in width. Further than that it starts to look ugly or out of proportion.

I love the subject and have been passionate with it since a kid my first encounter with the subject was on the wrong side of the law i.e. tagging and graffitti. But that was over and now am into the fonts and lettering design.

Definitely comment back on this if you have information.