Is it a driving safety hazard to drive with warped brake rotors?

I have a 2006 Chevrolet Cobalt LS Sedan with 93,000 miles. When I drive down a steep hill and press on the brakes, the steering wheel shakes and I can hear it,and when I drive fast on the highway the steering shakes but not as much as driving down a hill without pressing the brakes. I called the mechanic and he said it is warped brake rotors. I had my rotors changed in April 2011 and this problem has been happening for over a year. I am scheduled to see the front end mechanic to see if that is the problem and I may just get my rotors replaced or fixed. Your thoughts? The Mechanic said I should get with the last mechanic whom changed my brake rotors, but I will see when I get my cars front end checked and he said it's not an alignment problem, but a brake rotor problem? How much are titanium brake rotors?