In a course, running overtime inconveniences your students and shows disrespect for the class that follows you. In a professional presentation, running overtime can result in negative impressions of you among colleagues that may be making decisions whether to hire you or fund a project of yours in the future. Finishing early usually leaves the impression that you didn't have much to say. If it's true that you don't have much to say, it's probably better to finish early than to bore your audience by subjecting them to long explanations of obvious points. The best way to find out is to practice your lecture in front of an audience who can tell you when the things that seem obvious to you might require further explanation.
If you're presenting a lot of material it may be appropriate to distribute a copy of your slides to the audience before the start of your talk.
The greater the technological complexity of the medium that you choose, the more important it is to have a lower-technology backup. If your talk uses overhead transparencies, make sure you have notes so that you could give it using a blackboard. If your talk uses PowerPoint slides, you can print them out on transparencies as a backup. If your talk requires a network connection, bring a backup disk in case the network is down. If it's very important that you start on time, it may be worthwhile to carry a piece of chalk and/or a reasonably new whiteboard pen.