Guanajuato is a really safe city, police are always around, and people are very friendly to tourists. During the day, the entire city is safe, and as long as you stay in the downtown area, walking around alone or in small groups at night is safe as well. Anyplace you'd want to go out at night is downtown. It is far far safer than the average American college campus and doesn't have much crime directed at tourists.
Overall, Guanajuato is an incredibly safe city, but there is one caveat. Do not ever go running/walking around the Panoramica the beautiful road around the edge of the city in the mountains in the evening or early mornings. Many rapes, assaults, and muggings of foreigners have occurred in this area and some of the victims were out running with friends. Groups of men may be OK, but even several women together will not be safe. It is, however, safe to run in the city in the early mornings and there are a number of gyms where you can pay by the class. Several of the foreign exchange programs have apartments on or near the Panoramica and those houses often have large parties. In the populated areas it's fine at all hours, but since it's a loop with a nice view of the city, many foreigners decide to run around it at dawn and parts of it are much more isolated.
