I don't think that one general rule of leniency can be applied to all threads. Sometimes I let tangents slip by, sometimes I encourage them, and sometimes I remove them. There are many factors in my decisions, and I can explain them on a thread-by-thread basis but I cannot give you a general outline.
However, I can say that usually the only tangents I end up removing from Hydlaa Plaza are those that are comprised of more spam than decent content. When several people begin to go in circles in a meaningless one-line exchange, it's probably time to do some cleaning. If the thread drifts off on a natural evolution of a subject, chances are its okay. But again, it depends. The person who started a thread does have more say on its course than others, but moderators play the final card. If a thread is considered "open" but for two pages you're talking about the best way to make toast when the original topic was American Sign Language, that's not the sort of open I would consider acceptable.
I think to avoid complications, post as you usually do and through time take moderator actions into consideration.
Edited to add:
The purpose of the Hydlaa Plaza section is to open the floor to threads that are not related to the PlaneShift project. It does not mean that you have more freedom to make posts off-topic to a thread, and it would be a mistake to interpret it as such.