You didn't say which areas of the city you're going to be in, but assuming it's the typical central business and leisure areas like Reforma, Polanco, Zona Rosa, Condesa, and Santa Fé on the way to Toluca, I don't see why you should have any problems. I agree with cparekh, I've been working here for 2 months now, and see no difference in safety to any other major city.

Apart from the airport taxis, which seem to be very well regulated (just find a kiosk in the arrival area after exiting customs), you'll probably be taken for a ride in more ways than one: taxi fares are just random numbers. A huge amount of money is not at issue (normally less than USD 10 for a trip anywhere within a 15km radius). If you ask your hotel to call a taxi (usually the most expensive option), they'll probably register your trip and the taxi-drivers details if that makes you feel safer.

Other minor points: no trolleys in the airport after exiting customs, and traffic is an all-day nuisance (allow for it in your plans, or take the metro).