Can I just cut power to a central air and heating system, or should I follow a shutdown procedure?
Very few HVAC systems aren't able to handle abrupt power loss. I've flipped the switches on dozens of systems hundreds of times to change filters and perform other service. The gas valve will close and the control boards have algorithms to recover. 

It may feel nicer to shut them down gently, but there's no good reason to worry about it. After all, power does fail at times, and when have you heard of an HVAC disaster as a result?