No. Please do not wear high heels unless they are block heels or wedges. This 16th century city has all cobble stone streets. Most people who wear the wrong shoes end up hurting their ankles. If you are not comfortable with cobble stone streets please wear flats with traction.
While it is safe, San Miguel de Allende is a heavily populated city. Please be aware of your surroundings and take the usual precaution when in any urban environment.
It is around 6,200 ft above sea level. If you are prone to altitude sickness please bring Aspirin, Oxygen or consult with your doctor.
It is about 175,000
San Miguel de Allende is in the Central (US Central) time zone.
We would recommend having some pesos. Pesos are used in the cabs and markets. Most shops have a credit card machine, but not all of them. $1 US is somewhere between 18-20 Pesos, but check before you head to Mexico. ATMs are available, but they only dispense Pesos and sometimes have lines. You can change your dollars into Pesos in the airport, before heading to Mexico as well at the Cash Exchange kiosk.