I recently organized a symposium where a majority of the speakers exceeded the times they were allotted. It wasn't a close call, they exceeded their times by extensive amounts. The results were sessions that lasted too long and little time for questions and discussions. It is far from clear that the extra speaking time resulted in better talks, in fact probably the opposite. Going over one's time is often a symptom of not really deciding what the speaker wants to say and not understanding what the audience needs to hear.
Sticking to your time is a useful discipline. It is one tool for forcing the speaker to think through the purpose of the presentation. It is also a polite gesture to both the audience and the organizers of the talk.
The best way to insure that you stay within your time is to dry run your presentation. A dry run provides other advantages, as will be discussed in future "tips."
Remember, speaking faster to get through all your slides does nothing for the audience.