Closing off registers is generally not a good idea because it is very common for houses to have duct systems that are already only marginally large enough to handle the airflow from equipment that is often over-sized for the home's needs.

When you start closing registers, you restrict the airflow further, thereby reducing the efficiency of the equipment. This is especially a concern for A/C, since cooling capacity and efficiency are directly related to adequate airflow. A 3-ton unit can perform like a 2-ton unit if airflow is restricted.

Crawlspace vents: (assuming you mean vents to the outside) depending on the climate, it is generally good to open the vents in summer, but close them in winter.