The below is based on threads from other forums and discussions with people who have read the wording of laws and only applies within the US and the state of Virginia (California and Arizona specifically have special privacy laws (read movie star image protection laws).
The most important thing to remember is no matter what you do you can still be sued, however, paperwork will minimize your losses.
If nobody is recognizable you should also be okay. recognizable is defined as as somebody else being able to recognize them, not themselves by their rememberance of presence.
- First if you don't plan on making any money whatsoever from the images you should be okay as far as winning any law suits.
- Posting on the internet for your own portfolio use technically is okay. Posting for news worthiness is luckily still fully protected. (except in Arizona and California). Posting for advertising the image or a product with the image needs a model release.
- Entering a contest would probably depend on what they intend to do with the wining photos. If they intend to publish for advertising purposes you probably will need a model release. If all they do is show and announce the winner, you will probably be okay.
Note on Arizona and California: Both of these states apparently have "privacy" laws that protect people from photos taken when there is a reasonable expectation of privacy. The intention was to cover people using 600mm lenses from 3 miles away sitting on a hill and catching people in their backyard. In practice several movie stars (Striesand comes to mind) are abusing it to keep unwanted images out of the publics hands. Examples of bad use of law include some aerial photos taken of Malibu being used primarily for environmental and public access issues. Home owners do not own the beach in California and must allow access to the public which the movie stars obviously don't want just anybody visiting the beach, so naturally they object to people seeing if they have blocked public access to the beach. All this is currently in the US court system.
Sorry, personal pet peave injected above.
End summary,
If you are not making any money you should be okay, but someone could cause problems for you and that is when the paperwork helps to show intention of use.
Or so I have been told. Hope that helps.