Many speakers use humor at the beginning of their speech, because introductory humor can be a great attention getter. A funny opening sells both the speaker and the speech to the audience. Someone introducing Thomas Edison dwelled at length on the talking machine. When Edison was finally allowed to rise he said, "I thank the gentleman for his kind remarks, though I must insist upon a correction. God invented the talking machine; I invented the first one that can be shut off."
Humor, early in the speech, works well to establish a rapport, but only if it fits in well with your presentation. Too many speeches start out with humor for humor's sake, and the audience gets put off or sidetracked, instead of involved in your topic.
One of the best places to make a humorous point is in the title of your speech. Every title should make a point, and a little humor can make your audience anticipate the speech to come. You don't need to be matter-of-fact or dull when you can title a speech for telephone salespeople, "Why Am I Still on Hold?" or a speech on public speaking, "If I'm the Speaker, Why Is the Audience Snoring?" One of my favorite titles was for a speech on tax deductions. The accountant who presented the information called his speech, "Everything You've Always Wanted to Know About Charitable Deductions, but Were Too Cheap to Ask." If your talk is basically serious and has no other humor, you should avoid starting with humor. You don't want to raise false expectations.
The beginning isn't the only time that humor can add punch (or a punch line) to your presentation. You can use it at the end of your presentation as well, to provide an uplifting moment after a heavy or grave subject matter, or to raise spirits when needed. You can inject humor in the middle of a presentation at a significant transition or a change of subject—this is like a fresh beginning in some ways. But do not make humor the subject; merely use it as garnish!