The rules are very simple:
1. If the thread originator has chosen an answer you leave it alone, period.
2. If you come across an obviously abandoned thread (2 or more months) with no chosen answer you may choose one.
3. If you come across a thread with an answer that is obviously wrong, or a better one exists, you may change it, unless the answer was chosen by the thread originator, in which case you leave it as is.
4. You do not choose an answer in an active thread unless you are the thread originator.
The moderator team is quite good at spotting and dealing with abuse of the BA system, so be very careful with the power.