Stray dogs certainly have been known to bite people, but it really does come down to the individual characteristics of each individual dog. It's difficult to predict whether a stray dog may bite someone or not, but here are a few reasons why a dog may decide to bite:
Sometimes dogs bite because they're scared. Think of the human fight or flight concept and then apply that to dogs. Stray dogs are unlikely to understand whether a human being is a threat or not, and in fear, may decide to act out and bite.
Protection
Dogs can be very protective of their owners and property, and although stray dogs may not be protecting their owners, they may have a place that they feel they need to protect. If you enter that territory, the stray dog might bite you.
Mistreatment
If dogs have been mistreated by human beings, they may have a natural tendency to want to be aggressive to other humans. These traits can usually be trained out of the dog, however. You should have a look at the Dog Whisperer for cases like this!
