I noticed that even though I have general rules I follow, my own window closing behavior can be inconsistent, and the behavior of others can be quite different than my own as well, so I thought I'd inquire how others view this...
When I am using an e-mail or social site (facebook etc), I will log on, check messages, read what fiends are doing, then nearly always close the window.
When I am browsing forums, I usually open each post in a new window and when I am done reading/replying to the post, I will close the window, _unless_ it has some information I think I may want to re-read later in which case I will bookmark it then leave the window open until I'm ready to use the computer for something other than browsing. I often open multiple threads at the same time and I do not consider myself to be "done" with a thread until I have read as many messages as I want (or are available) and decided if I want to respond or not. I may or may not read the thread in chronological order
When I visit news or entertainment web pages, I will go there for the content and promptly close the web page when I'm done. If there is/was something I thought I might want to re-read or forward to a friend, I will bookmark it, and may copy the text of it to a local computer file (in case its not there when I come back).
If I get sidetracked by a phone call, or get the whim to do something else, then I may leave my browser windows open when I otherwise would have closed them (technically I'm not done using them, so I guess it is more consistent than I first thought).