San Miguel de Allende and the state of Guanajuato are generally free of the crime seen in other parts of Mexico. At this time, there is no safety advisory in effect for the city or the state. Expats should be able to travel anywhere within the area and feel safe.
The U.S. Department of State says that crime in Mexico varies widely by location with the worst crime reported in the states that border the United States and the south of Mexico, mostly because of drug cartel activity.
You can stay updated on travel warnings and safety information at this U.S. Department of State website.