The website How To Peru has posted a wonderful article recently about the importance of photography etiquette for those visiting Peru.  The author Tony Dunnell puts together a rather useful guide to taking photos in Peru and how it is important to always ask for permission or to check for restrictions before taking the shot. 

I will agree with Tony on the fact that in Peru it is important to try to ask an individual before attempting to take their photograph, especially if it is a portrait photo, however other times an opportunity presents itself that just can't be missed (this is where reward usually outweighs risk).  Just be prepared for the consequences that will follow your shot. 

Another good issue that Tony addresses is that of photographing military and police personnel in Peru.  In cities like Lima police officers (especially men) tend to shy from camera's especially when they are being photographed while "questioning" (note the use of quotes as this word is used lightly and can be left to your imagination.  Those who have or currently lived in Lima will understand) a driver.  Anywhere outside of Lima and unsolicited photography of police officers will more than likely get you in trouble (remember its just you and him out in the vast open, far from protection of the city.  Basically he/she has the upper hand in this situation).  As far as military personnel, I agree with Tony, always ask these uniformed individuals since their power and authority FAR surpasses that of most national police, except perhaps a high ranking police officer.