Young male gets bit on forearm while examining stray dog's ID tag. Should rabies prophylaxis be administered?

Depends on whether:

1. the dog was/is exhibiting rabid behavior; and
2. whether the dog is available for observation.

If the dog is neither acting rabid nor available for observation, administration of prophylaxis depends somewhat on the epidemiology of rabid dogs in the patient's locale (click picture to enlarge):

Rabid dogs reported in the US in 2008

Source

 DeMaria, A. and Techasathit, W.  "When to use rabies prophylaxis"  Up to Date.  Jan 2010.

"Rabid dogs reported in the United States during 2008."  CDC.  http://www.cdc.gov/rabies/resources/publications/2008-surveillance/dogs.html