I am not a lawyer. Following that disclaimer, about disclaimers:
The disclaimer is used to combat accusations of defamation. Defamation requires:
1. A defamatory statement, 2. Of and concerning the plaintiff, 3. Publication, and 4. Damages. (2nd Restatement of Torts Section 558). Damages are presumed in a libel action and presumably publication (technically communication to a 3rd party) is the goal. The disclaimer, therefore, only really affects the first two elements.
Essentially, the disclaimer tries to raise the burden of proof on these two elements by explicitly, and preemptively, stating the opposite. My story cannot possibly be about John Smith because I said it wasn't about anyone. Even if it were about John, which it's not, these things didn't actually happen anyway. Generally, it is more difficult to pass the 'of and concerning' element than the defamatory statement element, so I will concentrate there.
Whether or not a statement is of or concerning an individual is typically a question of fact left to the jury (Diaz v. NBC Universal). Effectively, this makes the question a toss-up. If there are a significant number of specific details, the jury could find defamation despite the disclaimer. For example, in Fetler v. Houghton Mifflin, the 2nd Circuit overturned a lower court ruling, finding there was enough of a question of identity to put the question to the jury where, "there are few, if any, other families with a minister father and thirteen children in which the third, fourth and eighth are girls and the eldest a son with great responsibility, who toured Europe in a bus in the 1930's giving family concerts." The disclaimer in that case was given considerable weight by the lower court, but was completely dismissed by the 2nd circuit because it wasn't included on the initial drafts sent to the publisher.
Some courts give the disclaimer a lot of weight. (Smith v. Huntington, dismissing a suit expressly because no reasonable jury could identify plaintiff in light of the disclaimer). Others give it less weight depending on wording, location, and other considerations. (Stanton v. Metro Corp, ignoring the disclaimer as the reasonable reader could miss reading it).
Include a disclaimer. Make it prominent (at the front is better than at the back, large font is better than smaller). At best, it will keep you out of court. At worst, it will be considered as a factor.
Include the disclaimer on every copy. "Publication" in the law means communication to a third party, which includes test readers, the publishing house itself, your family, or anyone who could identify the potential plaintiff and recognize the defamatory meaning of your statement.
Don't think the disclaimer makes you invincible. Even the best disclaimer will not help if you follow it with, "My neighbor, John Smith, at 123 Fake St., spends his weekends at his adopted nephew's lakehouse in Skagway torturing puppies." The more truth you include, the harder it will be for the jury to believe you when you claim fiction.
Defamation is State Law governed. All the aforementioned could go out the window depending on state law. This includes your state (presuming someone there reads it), the publishing house's state (presuming someone there reads it), and possibly any state where the story is distributed. If you are concerned, talk to a lawyer before you show the book to anyone. Any statements to your lawyer are covered by privilege, so hey, at least you've got one test reader.