I have Dropbox installed on my Mac with OS X Lion 10.8.2. I don't use it often, so I eliminated it from the login item list. However, every time I open Dropbox from the application folder, it automatically add itself to the login item list. Is that normal, even if I locked changes? Can I prevent Dropbox to add itself to my item list every time I open it?
Under Dropbox preferences, there is a checkbox for "Start Dropbox on System Startup".
However, as patrix says, Dropbox is really only useful if you leave it running all the time.
The idea of Dropbox is to always sync files across all your devices in the background. So disabling the login item doesn't really make sense from a Dropbox point of view. And at least at first glance I don't see significant drawbacks from keeping it there either.
If you just want to access some files in your Dropbox account you could use the web interface instead.