I love that you asked this question and I have met many people along the way with varying opinions on this topic. Here's My takes:
1. I wont break anything. Period. Not to gain access, not to create an aesthetic. If I get caught or questioned by police, I do not want them to be able to accuse me of more than trespassing. No breaking and entering and no vandalism for this guy.
2. I do not brings props for URBEX photos, not because of any "rules" per se, but I do not see a reason. I am just shooting what I find at the location. I am trying to create the best photo I can of the location I find, the way I find it.
3. I have however brought props for a portrait shoot like this stool and picture frame.
frame7 by
kabdad, on Flickr
4. I typically don't stage an urbex scene, but I have been known to move a chair, which I guess is still staging, but what I like to think of as a milder form. I have photographed rooms (like this classrooms) that was likely staged by others
5 by
kabdad, on Flickr
5. I do not take souvenirs or memorabilia from the sites I explore. If it was there when I arrived, I want to to be there for the next guy... (but I recently saw a photographer and her daughter pay a homeless guy haul a large chair out of a site and load it in their SUV!!!) To me this is similar to #1. If police see me leaving, I don't want to be accused of anything but trespassing. Not stealing, or theft, or anything else!!
Just my two cents.